Tentacool can be found in the following areas: Routes 4, 10, 11, 12, 13, 19, 20, 21, and 24, Treasure Beach, Kindle Road, Bond Bridge, Five Isle Meadow, Memorial Pillar, Water Labyrinth, Resort Gorgeous, Water Path, Green Path, Outcast Island, Tanoby Ruins, Pallet Town, Cerulean City, Vermilion City, One Island, Five Island, Icefall Cave and Trainer Tower (surfing)
Tentacruel can be found in the following areas: Treasure Beach, Kindle Road, Bond Bridge, Five Isle Meadow, Memorial Pillar, Water Labyrinth, Resort Gorgeous, Water Path, Green Path, Outcast Island, Tanoby Ruins, One Island, Five Island, Icefall Cave and Trainer Tower (surfing)
Unless you’ve seen Tentacruel in action, you wouldn’t really get how good it can be. Here’s a squid, and we can all see the unbelievers judging over there. You look at those stats and think, this is a specially bulky needlessly speedy Surf slave. But no. Tentacruel is, in theory, the best Surf user out there. It has so much special bulk, you’ll be left wondering how it’s living all those hits if you don’t know about it. Its Attack is good enough for you to slap physical attacks on without worrying too hard. Remember Pidgeot? This guy has the same offenses, but switched, and a much larger movepool with which to put them to good use. Oh, and remember Pidgeot, how it’s damn near everywhere? Tentacool is basically the Water equivalent of Pidgey. And you can’t say Rattata, because damn if Pidgeot isn’t final team level amazing, and Tentacruel isn’t equally carry. Speaking of carry, remember Pidgeot? Okay, that's enough, but Tentacruel can and will be the best thing you can Surf on. Forget Blastoise, forget Slowbro, forget Starmie, Lapras, Vaporeon, Dragonite. Tentacruel was destined to be a Surf user.
Fighting Dojo (Saffron City, Fighting-type): Despite the mediocre physical defense, Tentacruel can attempt to kinda tank the Fighting hits - they’re resisted. Hitmonchan’s elemental punches, on the other hand, become even weaker on that Special Defense.
Rival (Silph Co.): Surf out the resident Fire-type, you can also Ice Beam the Pidgeot if you have the option. You’re actually fine taking on Alakazam with Tentacruel, as its only attacking move is Future Sight, which doesn’t even take type into consideration in this generation.
Giovanni (Silph Co., Ground-type): Nidoqueen and Rhyhorn are both OHKOs. Nidorino isn’t that safe of a bet, but he can’t really deal major damage to Tentacruel. Kangaskhan will get through that defense of yours, however, and deal massive damage.
Gym #6 - Sabrina (Saffron City, Psychic-type): Don’t. It’s not worth it.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Here’s Tentacruel’s chance to shine. Seeing how it’s a very late game mon and the Fire gym is a very late game gym, there’s no question about whose Special Defense should be tanking all those Fire Blasts and dishing the hurt right back with powerful Surfs.
Gym #8 - Giovanni (Viridian City, Ground-type): Remember Pidgeot? In stark contrast to Pidgeot, everything BUT Dugtrio is easy pickings here. Surf handles pretty much everything, including the damn mole, but Dugtrio is just faster and can OHKO with the super-effective Earthquake, so it should be avoided.
Rival (Route 22, pre-Elite Four): Surf the resident Fire-type, again. Pidgeot, you should probably have Ice Beam by now. Rhyhorn, the new guy, will get OHKOed by a Surf without question.
Elite Four Lorelei (Indigo Plateau, Ice-type): You can’t really do much here without Giga Drain. If you need to tank a few special hits and heal, you can send out Tentacruel, otherwise no. Tentacruel is good for this role because most of Lorelei’s Pokémon are special attackers and carry resisted hits. Just stay away from Slowbro and Jynx. Those Psychics will hurt. You could try to out-stall Cloyster with Toxic, but a few Surfs should take it out too. Cloyster’s special bulk is absolutely pathetic.
Elite Four Bruno (Indigo Plateau, Fighting-type): Onix and Onix are no match for Surf. The rest can put up a bit of a fight if you let them. In fact, it’s very ill-advised to try, because Tentacruel’s Defense is its worst stat.
Elite Four Agatha (Indigo Plateau, Poison-type): Golbat drops to Ice Beam. Leave the others for someone with a super-effective attack, lest they start trolling you. In a pinch, though, Surf and healing items will work fine.
Elite Four Lance (Indigo Plateau, Dragon-type): Ice Beam dragons, Surf Aerodactyl. Who designed this freaking fight anyway? The only thing that won’t die to it is Gyarados. Take that out with someone else. Like an Electric-type.
Champion Rival (Indigo Plateau): Same as last rival fight. Take out the fire type with Surf, take out the Rhydon with Surf, take out the Pidgeot with Ice Beam if you want to. Unless you have a better thing planned, you can tank Blastoise’s hits and stall it with Toxic.
Tentacool’s level is entirely random, and can be between 5 and 40, so its moveset at the time you get it can and will be random. Such is the wonder of Surf encounters. Tentacool starts out with Poison Sting, and gets the ever-annoying Supersonic at level 6. 12’s Constrict is entirely useless. Just you wait, at level 19 you get Acid, which will be your go-to Poison STAB until postgame. Level 25 has Bubblebeam, but to be honest, you’ll have the Surf HM on hand anyway, so why bother. At level 30, Tentacool, or Tentacruel depending on how lucky you were with levels, gets Wrap. Formerly famous for its brokenness, now... not. Level 38 has Barrier in store, the only real use of which is to buff up Tenta’s worst stat. By the time you reach Screech, you should be evolved already, so there’s no point in saying that Tentacool learns it earlier than Tentacruel’s 47. Level 55 holds Tentacruel’s final attack, the less-than-accurate Hydro Pump, Fire Blast’s missing brother.
From TMs, you’ll want TM13 Ice Beam and maybe TM06 Toxic, and if you’re going to delve into the post-game, TM36 Sludge Bomb. If you’ve kept it, you can slap on TM19 Giga Drain to make Tentacruel more useful against its Water brethren. Another obvious mention goes to HM03 Surf, because frankly, if you don’t know you should slap Surf on Tentacruel, you shouldn’t be allowed to nuzlocke.
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.
Tentacool Line Ratings
What Nature do I want? I’ll be honest, pretty much any. You can afford to sacrifice a bit of Speed, or Attack, for the benefit of other stats. Anything that lowers Defense isn’t too much of a liability, since you won’t exactly be sending Tentacruel out to tank physical hits. The only necessarily bad nature is any that lowers Special Attack. So pretty much, Modest and Quiet are amazing, whereas Adamant and Impish are crap.
Which Ability do I want? Sadly, neither of them sees much use. If you plan on running physical moves, Clear Body is better so you can avoid the Intimidate reductions, but otherwise, it won't make that big a difference, if at all.
At what point in the game should I be evolved? By the time you get Tentacool, you’ll be in the range of the level curve where turning it into Tentacruel is a necessity, if you’re planning on using it. And let’s be honest, why wouldn’t you?
How good is the Tentacool line in a Nuzlocke? Get the hell out of town. This thing is a freaking beast. You’re not going to not get one, and it’s super worth using for how common it is. Tentacruel will tank for you to hell and back, it’ll carry you through the late-game, and overall Surf you pretty well. Plus, it can Toxic stall. Despite its relatively common weaknesses, it’s definitely worth using.