Wishiwashi can be found in Brooklet Hill as a static encounter. It can also be found by fishing in the following places: Routes 7, 8, 9, 13, 14, 15, Akala Outskirts, Brooklet Hill, Kala'e Bay, Melemele Sea, Secluded Shore.
Just as you should never judge a book by its cover, you should never judge a fish by its Solo Form. Outside of battle, Wishiwashi looks like a waste of a party slot: it has the lowest base stat total of any Pokémon ever created, it has a somewhat shallow movepool, and its wimpy look suggests that it'd be outclassed by even the Magikarps and Sunkerns of the world. Get it up to Level 20 and toss it into a battle, though, and you'll see why this thing is able to scare even Gyarados away. Whenever Wishiwashi's HP is above 25%, it changes into its School Form, which has a base stat total of 620 (higher than that of most legendaries). Though its HP and Speed are rather lackluster at 45 and 30 respectively, all of its other stats are absolutely incredible: 140/140 offenses allow it to hit extremely hard with both physical and special moves, while its 130/135 defenses help keep it in School Form for as long as possible. Although its level-up learnset is rather underwhelming, Wishiwashi has some tremendous TM options, including Scald, the TM for which you can pick up right after catching Wishiwashi. Despite its offensive and defensive prowess, though, Wishiwashi is not a perfect Pokemon: it's annoying and expensive to have to keep healing off any chip damage it suffers, and it essentially becomes useless for the rest of the battle if it ever reverts to Solo Form. Additionally, its low Speed can occasionally prevent it from being the monster that it should be. By no means does this mean that you shouldn't use Wishiwashi, though; its positives far outweigh its negatives, and it can be a phenomenal member of your team if you use it right. Despite its flaws, it's arguably one of the best Pokemon you have access to in the early-game, and it fits well on any team composition.
Totem Wishiwashi (Brooklet Hill): You should have Scald at this point, whereas the Totem's strongest move is Water Gun. Provided you don't have terrible luck, this should be a relatively easy fight for Wishiwashi to solo. If it calls a Wishiwashi (Solo Form) as an ally, feel free to ignore it, but if it calls an Alomomola, try to defeat it first so that it can't spam Heal Pulse on the Totem Wishiwashi.
Battle Royal (Royal Park): Even if everyone focuses on Wishiwashi, it won't even come close to dying. Use Scald on Kukui's Rockruff to end the battle.
Totem Salazzle (Wela Volcano Park): Wishiwashi is one of the best Pokémon you can have for this battle. None of Salazzle's attacks can really do anything to you, while a Hydro Vortex off of Scald should be an easy OHKO. Mystic Water is also usable, but as it generally makes Scald only a 2HKO, the Waterium-Z is the preferred item for this battle.
Totem Lurantis (Lush Jungle): Keep Wishiwashi away from this battle. It may have the stats of a legendary, but even Kyogre wouldn't like taking a STAB Solar Blade off of 105 base Attack to the face. If you end up taking out the Lurantis before its ally Castform or Trumbeak, though, Wishiwashi is more than capable of switching in and finishing off the battle.
Plumeria (Akala Outskirts): Golbat's Confuse Ray is annoying, but it's not dangerous in the least. Scald should be able to either OHKO or 2HKO it, depending on what item Wishiwashi is holding. Salandit is going down to a single Scald no matter what (unless you're in Solo Form, in which case you should immediately switch out).
Olivia (Ruins of Life, Rock-type): Nosepass and Boldore won't go down immediately due to Sturdy, so don't waste your Z-Move on them. Neither pose much of a threat to Wishiwashi, but both can be rather irritating: Nosepass has Thunder Wave and Spark, while Boldore has Mud Slap. Lycanroc isn't too big of an issue since it'll go down to Scald; just make sure Wishiwashi is near full health so that it can tank the Lycanroc's Z-Move.
Lana (Konikoni City, Water-type, optional): Stay away from Chinchou, for obvious reasons. The Thunder Wave + Electro Ball combination will do quite a bit, and isn't something you'll want to risk. Shellder and Araquanid are relatively easy pickings for Wishiwashi, since neither can do much damage to School Form. You'll want to use a physical move on Araquanid since it resists Scald, is immune to burns due to Water Bubble, and has good Special Defense. Feint Attack and Take Down both work well, with Feint Attack being preferable due to its perfect accuracy and lack of recoil.
Kiawe (Royal Avenue, Fire-type, optional, Sun only): Spam Scald and win.
Mallow (Lush Jungle, Grass-type, optional, Moon only): Don't use Wishiwashi in this battle unless you really have to. Mallow's Grass-types won't actually do all too much damage to Wishiwashi thanks to its titanic bulk, but since they all resist Wishiwashi's STAB, you won't be able to hit back very hard.
Nihilego (Aether Paradise): Two Scalds should take this Ultra Beast down. Nihilego doesn't have anything that can severely damage Wishiwashi, so this matchup should be quite safe.
Hau (Malie City): Keep away from Raichu, for obvious reasons. The same goes for Leafeon and Dartrix. Wishiwashi can easily handle anything else Hau could potentially have, though.
Totem Vikavolt (Hokulani Observatory): Using Wishiwashi in this battle is a bad idea, for obvious reasons. Wishiwashi will probably be able to tank a few Sparks once the Charjabug has been defeated, though.
Guzma (Malie Garden): Golisopod is a serious threat; First Impression off of 125 base Attack is no joke. However, since it can only use it on its first turn and its only other attacking move is Razor Shell, Wishiwashi should be able to wall it following Turn 1. Make sure you don't let it set up too many Swords Dances, though.
Totem Mimikyu (Thrifty Megamart): This fight is tough. Mimikyu takes no damage from your initial attack due to its ability, Disguise, so make sure not to waste your Z-Move. It also has rather decent Special Defense, so it's highly likely that a Scald-based Hydro Vortex won't knock it out from full. Due to its lower Defense, however, it may go down to a Dive-based Hydro Vortex (since it's a Z-Move, the attack only takes one turn instead of two). If you can, try to take down the ally Haunter/Gengar as soon as possible, since they're fast and pack Hypnosis.
Plumeria (Route 15): Same as last time, really. Golbat is still annoying, but it doesn't pose a serious threat. Salazzle should go down to Scald or Bulldoze.
Guzma (Shady House): Again, watch out for First Impression. Everything else should be treated like the last battle, as his team hasn't changed.
Gladion (Aether House): Wishiwashi walls everything on his team and does massive damage with Scald. Type: Null is the biggest threat, but it isn't powerful enough to 2HKO Wishiwashi from full health.
Nanu (Malie City, Dark-type): Scald does massive damage to Sableye and will likely be able to score an OHKO. The same goes for Krokorok. Persian is a slight issue as its Z-Move is quite powerful, but a healthy Wishiwashi should be able to eat it up and retaliate with either a Scald, Hydro Vortex, or U-turn.
Faba (Aether Paradise): Hypno loves its Hypnosis/Nightmare combination, so either equip a Chesto Berry or stock up on Awakenings. If it doesn't go for Hypnosis (or goes for Hypnosis and misses), this should be a very straightforward battle.
Faba (Aether Paradise, tag battle with Hau): All of Faba's Pokémon are Psychic-type, so U-turn is your friend here. If you have a Dark-type, take advantage of Wishiwashi's slow U-turn to give it a safe switch-in.
Guzma (Aether Paradise): Handle Golisopod and Ariados as you did last time; their moves haven't changed. Masquerain is somewhat dangerous, as it hits hard and can flinch you with Air Slash. Wishiwashi can easily handle Pinsir, but a Bug-resist would be a better choice if you want to minimize risk.
Lusamine (Aether Paradise): A healthy Wishiwashi can handle everything on Lusamine's team except for her Lilligant. Clefable can't touch Wishiwashi unless it gets a very lucky Metronome roll. You'll need coverage moves to take down Milotic, but it can't do much back to Wishiwashi. Bewear and Mismagius go down quickly to Scald and Aqua Tail, respectively.
Hapu (Vast Poni Canyon, Ground-type): Spam Scald until your HP is low. Gastrodon and Flygon should both be 2HKO'd by Mystic Water-boosted Scald, while Dugtrio and Mudsdale should both be one-shotted.
Totem Kommo-o (Vast Poni Canyon): Use something else for this battle. You don't have access to the Ice Beam TM by this point, so Wishiwashi can't do much to Kommo-o. It can easily handle Scizor once the Kommo-o's gone down, though.
Lusamine (Ultra Space): Use Aqua Tail on Clefable, as as its bulkier on the special side and also gets a Special Defense boost when it comes in. Milotic should be taken on by something else, as it has Recover and Wishiwashi isn't likely to be able to 2HKO it with a coverage move. Mismagius should go down to two Aqua Tails; the worst it can hit you with is STAB Shadow Ball, which shouldn't 2HKO Wishiwashi unless it gets a Special Defense drop. Bewear hits hard with Hammer Arm and has massive physical bulk, so use Scald to take it down. Lilligant should be handled by one of your other team members.
Solgaleo (Altar of the Sunne, Sun only): Metal Burst shouldn't be an issue, as it doesn't have negative priority and Solgaleo should always be able to outspeed Wishiwashi. Solar Beam is easy to watch for; if it starts charging it up, make sure to switch out to something that resists Grass. Solgaleo's Special Defense is slightly lower than its Defense, so use Scald over Aqua Tail for extra damage.
Lunala (Altar of the Moone, Moon only): Lunala is bulky and strong, but Aqua Tail will do quite a bit to it. Refrain from using Scald, as it has Mirror Coat.
Gladion (Mount Lanakila): Wishiwashi is able to take on every member of Gladion's team sans a potential Grass Silvally (only relevant if you chose Popplio). Wishiwashi is one of the best Pokémon to use against his Lucario, since it packs the Steel-type Z-Move Corkscrew Crash, which Wishiwashi resists. Aqua Tail handles everything, though you'll want to switch to a coverage move if Water Silvally or Grass Silvally comes out.
Hau (Mount Lanakila): Stay away from Raichu, Leafeon, and Decidueye. Watch out for Komala as well, as it packs a surprisingly powerful Wood Hammer, which can easily 2HKO Wishiwashi. Only take on Primarina if you have a strong physical move that hits it neutrally, such as Return; the same goes for Vaporeon. Flareon and Incineroar obviously go down to Scald/Aqua Tail (Aqua Tail hits Flareon much harder if you're debating which one to use).
Elite Four Hala (Pokémon League, Fighting-type): Wishiwashi can certainly soak up hits in this battle, but don't expect it to take too many of them. STAB Close Combats and Cross Chops will put a dent in your school of fish, so make sure you come equipped with plenty of healing items or plenty of Fighting switch-ins. Scald is your friend in this battle, as its 30% burn chance may well come in handy.
Elite Four Olivia (Pokémon League, Rock-type): Wishiwashi can easily handle everything on Olivia's team except for her Golem and Lycanroc. Golem is part Electric-type, has Sturdy, and packs STAB Thunder Punch; stay away from it if at all possible. Lycanroc has a brutal 270 base power (STAB included) Continental Crush, so make sure your Wishiwashi has full health or has some defense boosts if you plan on using it against Lycanroc.
Elite Four Acerola (Pokémon League, Ghost-type): Dhelmise is an obvious no, but everything else on Acerola's team can be handled by Wishiwashi. Scald is preferable to Aqua Tail in this battle, since Palossand is bulkier on the physical side and has Water Compaction, which sharply raises its Defense whenever it gets hit with a Water-type move.
Elite Four Kahili (Pokémon League, Flying-type): Wishiwashi does very well in this battle. Skarmory and Oricorio are both easily handled by Scald, while everything else takes massive damage from Ice Beam. Watch out for Toucannon, though; Bullet Seed off of a 120 base Attack stat is not something Wishiwashi's going to want to take.
Professor Kukui (Pokémon League): Decidueye and Magnezone are obvious nos. Lycanroc should go down to a single Scald/Aqua Tail. Ninetales isn't much of a threat; two Aqua Tails should be able to dispatch it. The same goes for Braviary, but use Ice Beam instead of Aqua Tail. Snorlax vs. Wishiwashi is a battle of the tanks, but Snorlax shouldn't be able to take more than two or three Aqua Tails. Primarina should go down to two or three Returns, but watch out for Moonblast, as it'll hit harder than you think. Incineroar should go down to one or two Scalds/Aqua Tails (depends on Wishiwashi's item, Nature, EVs, etc.) or one Hydro Vortex.
Tapu Koko (Ruins of Conflict): Nope. You just got Wishiwashi into the Hall of Fame, is this really how you want it to die?
When you get Wishiwashi, it should have Helping Hand, Feint Attack, Brine, and Aqua Ring. Tearful Look comes at Level 22, but it's really not all too useful. Take Down at 25 is usable, but the recoil makes it a somewhat unwise choice, especially considering the mechanics of Schooling. Dive comes at Level 30 as Wishiwashi's first physical Water move; it's actually pretty useful, especially since it provides you with the option of using a physical Hydro Vortex. Beat Up comes at Level 33; you may want to use it over Feint Attack, but Feint Attack is overall the more reliable move. Aqua Tail at Level 38 is a nice addition; it's generally superior to Dive due to its higher power and lack of a charge turn. The highly risky Double-Edge comes at Level 41; it's definitely usable, but you'll have to be very careful with managing Wishiwashi's HP if you decide to use it. Soak and Endeavor are useless outside of very special cases; don't waste moveslots on them unless you have a very specific plan in mind. Hydro Pump is the last move that Wishiwashi learns naturally, coming at Level 54. Despite its high power, you'll be better off using Surf or Scald as your main STAB move due to their 100% accuracy.
Right after you catch Wishiwashi, you'll pick up the Scald TM. This move will potentially remain on its moveset for its entire life; teach it as soon as you get it. Bulldoze comes near the middle of the game and gives good coverage; its side-effect of lowering Speed always comes in handy and somewhat makes up for the move's low power. U-turn is another great choice for coverage. It allows Wishiwashi to hit Grass-, Dark-, and Psychic-types hard, while also potentially giving one of your frailer teammates a free switch-in (opponent uses a move, Wishiwashi U-turns out to Salazzle, for instance). Rain Dance is worth consideration, but Wishiwashi is usually powerful enough without it. Return is always a good option, as it provides Wishiwashi with a good way of hitting opposing Water-types for lots of damage. Ice Beam is a staple on the moveset of practically every Water-type, and should definitely be given to Wishiwashi the moment you get the TM for it. Last but not least, Earthquake is a fantastic coverage move that should overwrite Return or Bulldoze when you get the TM for it; unfortunately, that won't happen until after you've beaten the game. The TMs for Surf and Waterfall come during the postgame and are both perfectly capable of serving as Wishiwashi's main STAB; the nice thing is, you can change between Scald, Surf, and Waterfall depending on which one fits the situation best. It also works to have both Scald/Surf and Waterfall on the same moveset so that Wishiwashi can hit from both sides.
Pokémon that can handle Electric-types: Wishiwashi is weak to Electric and is outsped by most Electric-types, so it's always good to have a Pokémon that either resists or is immune to Electric on your team.
Good Pokémon that fit this description include, among others: Marowak, Decidueye, Mudsdale, Golem
Pokémon that can handle Grass-types: Wishiwashi's other weakness. Ice Beam and U-turn can hit Grass-types hard, but taking a STAB Giga Drain to the face isn't something Wishiwashi will ever want to do.
Good Pokémon that fit this description include, among others: Toucannon, Incineroar, Ribombee, Decidueye
Frail, offensive Pokémon: Wishiwashi's slow U-turn is a massive boon to frail sweepers that don't like to take hits. NOTE: this recommendation assumes that your Wishiwashi knows U-turn.
Good Pokémon that fit this description include, among others: Salazzle, Jolteon
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
What Nature do I want? Anything, really. Wishiwashi's stats are monstrous enough that small changes won't make much of a difference.
At what point in the game should I be evolved? It's not technically an evolution, but make sure you get Wishiwashi up to Level 20 as soon as possible. Don't use it at all until it gets there.
How good is Wishiwashi in a Nuzlocke? Truly excellent. Its stats are incredible, and it absolutely destroys the early-game while staying very good all the way to the end. Thanks to its ridiculous bulk and attacking stats, it's unlikely that you'll often - if ever - see it revert to Solo Form.