Popplio is the Water-type starter. It can be received from Hala in Iki Town.
Despite all the good water types in Alola, Popplio still manages to stand near the top. With the amazing typing of Water/Fairy, an amazing movepool late game, and solid all around stats, Primarina can make a valuable ally, and you won't regret picking it.
Hau battle #1 (Iki Town): Spam Water Gun. You can only lose if you try to.
Hau battle #2 (Iki Town): Water Gun will kill Pichu before it can kill you with Thunder Shock, and Litten is a joke.
Ilima (Hau'oli City, Normal-type): Yungoos can only 4HKO you with Tackle, but be careful if it uses Leer to much. Smeargle can only 4HKO with Leafage as well, but be careful of crits, and bail if Yungoos used Leer.
Totem Gumshoos/Raticate (Verdant Cavern): Totem Gumshoos is technically possible as Gumshoos and Yungoos can't do much to Popplio at first, but they will double team you with Bite and Hyper Fang after a few Leers. Gumshoos also has Super Fang to take you to half from full HP. Totem Raticate is even worse, as it has STAB Bite, will potentially have Hustle, and will call in Rattata. To top it all off, they all have Leer. You might be able to Baby Doll Eyes and maybe Disarming Voice the Raticate if you're playing Moon, but Popplio can't help all that much during this fight.
Hau (Route 3): Pichu has evolved now so stay away. Litten is still a joke however.
Hala (Iki Town, Fighting-type): Mankey can only 3HKO with a crit Karate Chop so feel free to set up Work Ups or Echoed Voices on it. Be careful of Makuhita's Fake Out if you've been weakened, but Makuhita should be easy after that. 5 hits of Arm Thrust with no crits only might not even 2HKO depending on damage rolls and stats. Disarming Voice is only a guaranteed OHKO on Crabrawler at +4 and max power Echoed Voice only OHKOs at +1. All Out Pummeling does 70% at +0 and OHKOs at +1. Unless you managed to set up a lot Crabrawler should be avoided.
Ilima (Hau'oli City, Normal-type, optional): Avoid Gumshoos as it will kill you before you kill it, and has Pursuit so you can't just come in to take the Z-Move. Smeargle is possible with healing as you both 4HKO, but Smeargle is faster.
Hau (Paniola Town): Avoid the Pikachu and kill the Torracat. Even with its Z-Move, Torracat is no match for you.
Gladion (Route 5): Zubat is pathetic, and Type: Null is only possible with healing due to a lack of good water STAB.
Totem Wishiwashi (Brooklet Hill): The best thing you can do is burn things with Scald. Sit this one out.
Battle Royal (Royal Park): Hydro Vortex OHKOs Rockruff, and Protect is useless on Z-Moves.
Totem Salazzle (Wela Volcano Park): You actually have a chance to OHKO with Hydro Vortex of Scald. Salazzle can't kill you before you kill it.
Totem Lurantis (Lush Jungle): 0 Attack Lurantis Solar Blade vs. 0 HP / 0 Defense Brionne: 98-116 (140 - 165.7%) -- guaranteed OHKO. Lurantis will outspeed, and itemless Acrobatics will only 2HKO. Brionne needs to sit this one out.
Plumeria (Akala Outskirts): Hydro Vortex and Scald can beat Golbat before it beats you. Salandit is pathetic, but be careful of multiple crits off of Double Slap.
Olivia (Ruins of Life, Rock-type): With healing Brionne can solo her. Be careful of Rock Blast and Spark from the Boldore and Nosepass, and heal up to full so you can take Lycanroc's Continental Crush. Use Hydro Vortex on Lycanroc because the other two have Sturdy.
Lana (Konikoni City, Water-type, optional): Avoid Chinchou for obvious reasons. You hard wall Shellder so you can kill it with whatever. Araquanid does a lot of damage with Bug Bite and Hydro Vortex, but itemless Acrobatics can 2-3HKO, so fighting it is possible, but not recommended.
Kiawe (Royal Avenue, Fire-type, optional, Sun only): Spam Scald he really can't do much.
Mallow (Lush Jungle, Grass-type, optional, Moon only): Only fight Phantump because it doesn't actually have any grass moves for some reason.
Nihilego (Aether Paradise): Just use Hydro Vortex and kill it.
Hau (Malie City): Avoid Raichu and Leafeon for obvious reasons. Torracat is still okay though, so Scald or Hydro Vortex it.
Totem Vikavolt (Hokulani Observatory): If Brionne is your best bet for this fight your team has some problems.
Guzma (Malie Garden): Protect turn one on Golisopod and then either use itemless Acrobatics, Scald, or Disarming Voice to whittle Golisopod down. Be careful if it uses Swords Dance a lot though, and use Emergency Exit to your advantage. Ariados is a pushover as long as you're careful with Sucker Punch. Heal on it so you can take on Golisopod later.
Totem Mimikyu (Thrifty Megamart): Evolve or you can't do anything here. Mimikyu can 2HKO you with Play Rough, and Haunter will put you to sleep so you can't do anything. Be careful if it Mimics Sparkling Aria. Primarina doesn't like Mimikyu, but granted nothing likes Mimikyu.
Plumeria (Route 15): Oceanic Operetta or Hydro Vortex the Golbat and Sparkling Aria the Salazzle. Plumeria is helpless against Primarina.
Guzma (Shady House): Same deal as last time except Ariados is less of a threat.
Gladion (Aether House): Golbat can be handled by Psychic, Sneasel is easily handled by Disarming Voice, and Type: Null is beaten by your Z Move.
Nanu (Malie City, Dark-type): Disarming Voice the Sableye as it can't do much with Shadow Ball, Sparkling Aria the Krokorok, and Oceanic Operetta or Hydro Vortex the Persian.
Faba (Aether Paradise): Never Ending Nightmare OHKOs.
Faba (Aether Paradise, tag battle with Hau): You 2HKO with Shadow Ball his entire team. Slowbro can only put you to sleep with Yawn and Psychic you for a 4HKO. Hypno can hit you for a 3HKO with Psychic and put you to sleep, and Bruxish 2HKOs you with Psychic Fangs. Never Ending Nightmare one and leave the rest for a teammate.
Guzma (Aether Paradise): Same as last time for the first two. Masquerain can't do much damage with Air Slash so Sparkling Aria it. Pinsir can only 4HKO with X-Scissor, but be careful if your defense dropped.
Lusamine (Aether Paradise): Clefable can't touch you, but be careful if it Cosmic Powers or because of Metronome shenanigans. Avoid Lilligant for obvious reasons. Milotic can only hit you with Flail so Disarming Voice it. Mismagius falls to Shadow Ball, but be careful if it Mystical Fires too much. Psychic will kill Bewear.
Hapu (Vast Poni Canyon, Ground-type): Grab Energy Ball and OHKO everything with your various coverage.
Totem Kommo-o (Vast Poni Canyon): Moonblast OHKOs, and Kommo-o can't OHKO you with anything.
Lusamine (Ultra Space): Avoid Clefable, because with the Special Defense boost, it's to strong. Mismagius is taken out by Shadow Ball, but if it Mystical Fires too much, other match ups are impossible. Avoid Lilligant for obvious reasons. Milotic can still only hit with Flail so Moonblast away. Bewear falls easily to Moonblast.
Solgaleo (Altar of the Sunne, Sun only): Solgaleo can 2HKO you with Sunsteel Strike, and can boost its defense with Cosmic Power.
Lunala (Altar of the Moone, Moon only): Shadow Ball does decent damage to Lunala after Shadow Shield is gone. Moongiest Beam 2HKOs though.
Gladion (Mount Lanakila): Avoid Crobat and Silvally for obvious reasons. Weavile is frail, so just Moonblast it and call it a day. Lucario can't do much to you, even with Corkscrew Crash, so use your water STAB on it and call it a day.
Hau (Mount Lanakila): Avoid Raichu and Leafeon for obvious reasons. Incineroar can just be killed with your water STAB. Only fight Komala if you use your Z-Move on it as otherwise it does too much damage with Wood Hammer.
Elite Four Hala (Pokémon League, Fighting-type): Primarina literally resists everything his pokemon can do except for Hariyama's Fake Out. Just spam Moonblast.
Elite Four Olivia (Pokémon League, Rock-type): Water STAB everyone but Golem because of Sturdy. Be careful when Lycanroc comes in because of Continental Crush. Defense lowering natures should sit Lycanroc out as it will OHKO them.
Elite Four Acerola (Pokémon League, Ghost-type): Moonblast Sableye and call it a day. Drifblim should be avoided because of Amnesia and Baton Pass. Avoid Dhelmise for obvious reasons. Finally you can take on Froslass and Palossand with ease.
Elite Four Kahili (Pokémon League, Flying-type): Skarmory can't touch you, but it can set up Spikes so be careful if it sets up too many of them. Avoid Crobat for obvious reasons. Oricorio and Mandibuzz can be taken out easily by your STABs. Finally avoid Toucannon because of Bullet Seed.
Professor Kukui (Pokémon League): Sparkling Aria the Lycanroc, but it will probably set up Stealth Rock so use something faster if available. Avoid Magnezone and Decidueye for obvious reasons. Ninetales can't do much with Dazzling Gleam so Water STAB or Moonblast away. Ice Beam should kill Braviary if it uses Brave Bird, but if it uses Crush Claw then you have to switch out. Finally avoid Snorlax as its just too bulky.
Tapu Koko (Ruins of Conflict): NO!
Post-Game: Primarina is still good in the post game, and can hard wall some post game bosses.
Popplio starts with Pound, Growl, and Water Gun. Not the best, but it will do for the early game. At level 8 Popplio gets Disarming Voice. Hope you like it because you don't get any other Fairy STAB until Moonblast at level 44. Baby Doll Eyes is at level 11, not that good, but better than Growl. Next is Aqua Jet at level 15. Might be decent for priority, but will be out shined later on. At level 19, Brionne gets Encore. It's niche at best, so I'd pass on it. At level 24 you get Bubble Beam, pass on it because you have Scald by now. Sing can be nice, but it's not the best. Before evolution, Brionne gets Double Slap, worthless, so move on. Primarina gets Sparkling Aria, which is basically Surf that turns into a 195 power nuke with a Z-Move. It's up to personal preference whether you want over Scald or not. Hyper Voice comes at level 38, good, but your coverage moves are better. Finally you get Moonblast at level 44. If you don't want this you must be crazy. I guess Captivate, Hydro Pump, and Misty Terrain are there, but they're unnecessary.
Primarina gets a surprisingly wide TM movepool. Hidden Power can be good if you get a good type. Ice Beam is amazing, but it comes very late. Psychic, Shadow Ball, and Energy Ball make good coverage, take your 2 picks and you're golden. Acrobatics can be good early game, but you can't run Oceanic Operetta and it so pass later on. Finally, Light Screen and Reflect can be good for support.
Oceanic Operetta is your best Z-Move, as a 195 power nuke is amazing. Twinkle Tackle can be good secondary STAB in some cases. Never Ending Nightmare, Bloom Doom, Shattered Psyche, and Subzero Slammer can make good coverage moves as well.
Recommended moveset: Sparkling Aria, Moonblast, and any two out of Psychic / Shadow Ball / Energy Ball / Ice Beam
Fast Pokemon: Primarina is the slowest of the starters with only 60 base speed. Considering the fact that Pokemon in Alola are slow in general, they can outspeed and kill some things that Primarina can't.
Physical hitters: Primarina doesn't hit hard on the physical side. A good psychical hitter can break some of the special walls that Primarina can't deal with.
Good Pokémon that fit this description include, among others: Snorlax, Arcanine, Crabominable, Lycanroc, Salamence, Stoutland, Mudsdale, Marowak, Tsareena, Zygarde, Metagross, Electivire, Mimikyu, Aerodactyl
Ground-types: Hitting two out of three of Primarina's weaknesses isn't bad, and many of the ground types are neutral to Grass anyway. Primarina can also take care of Ice and Water types that they can't deal with anyway.