In both games, Krabby can be found on Routes 4, 10, 11, 12, 13, 19, 20, 21, 24, Cerulean City, Vermilion City, and Seafoam Islands by fishing with a Good Rod; a Super Rod may also be used if in LeafGreen. In LeafGreen it may additionally be found by surfing in Seafoam Islands, as well as by fishing with the Good or Super Rod on Treasure Beach, Kindle Road, Bond Bridge and One Island.
Kingler can only be found in the wild in LeafGreen on Routes 19, 20, and 21, Kindle Road, Treasure Beach, Bond Bridge and One Island by fishing with the Super Rod
Like most regions, Kanto offers a wide array of Water types, and unfortunately Kingler tends to hang out near the back of the pack. Low Special Attack makes it a poor choice at standard Water-type fare (namely Water and Ice coverage) and while its Defense is impressive its physical tanking capabilities are hindered by its meager HP. True, it can hit hard physically when it gets the chance...but is that really worth a spot on your team?
Gym #3 - Lt. Surge (Vermilion City, Electric-type): In the words of Crasher Wake, "Run from electricity!" Seriously.
Gym #4 - Erika (Celadon City, Grass-type): Three Giga Drain carriers = one dead crab. Keep off the grass!
Gym #5 - Koga (Fuchsia City, Poison-type): Surf against the Koffing twins, but Hyper Cutters beware - Kingler can survive a regular Self-Destruct but not a crit. Avoid Muk as a single Acid Armor will allow it to tank you to death. Kingler should be able to Surf Weezing into oblivion barring Smokescreen hax.
Fighting Dojo (Saffron City, Fighting-type): Return spam will take care of Hitmonlee or Hitmonchan; taking on both of them will require healing though.
Rival (Silph Co.): Pidgeot's low-power moves make it a perfect target for Return/Rock Tomb, though Shell Armorers should switch to Surf if it get a Featherdance off. Alakazam only has Future Sight so have at it. Exeggcute should probably be avoided; its Confusion is surprisingly strong against Kingler's poor Special Defense, plus it has plenty of other annoying moves. Also avoid Gyarados as Kingler can't survive Dragon Rage spam. Drown Growlithe with Surf. Only fight Charizard if Kingler has Rock Tomb, otherwise even resistance might not be enough to prevent death-by-Flamethrower. If you can outspeed Blastoise go ahead with Return, otherwise its a very close matchup (though if healing's allowed you'll be fine regardless.) Venusaur's off-limits for obvious Grass-flavored reasons.
Giovanni (Silph Co., Ground-type): Mud Shot/Dig makes quick work of Nidorino. Surf drops Rhyhorn so fast it's hilarious. Kangaskhan's going to be hitting too hard with Mega Punch to be taken on without healing (unless it misses at all, in which case carry on). Barring paralysis hax (and crits for Hyper Cutter Kinglers), Nidoqueen can be taken out with some Surf usage.
Gym #6 - Sabrina (Saffron City, Psychic-type): Kadabra should go down in one Return, though be aware that a crit Psybeam could be lethal. Avoid Mr. Mime; while fragile, there's still a good chance of it scoring a kill first, particularly if it uses Barrier. Venomoth's safe to fight; STABless Psybeam is not that big a threat. And don't fight Alakazam, as Kingler's much too specially frail for that.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Surf (which despite Kingler's offensive stat gap is still more powerful than any physical move here) will do well against everything except the too-strong Arcanine. Note however, that it'll be unlikely to handle them all without healing and that a Hyper Cutter Kingler will be in danger at all times from crit Fire Blasts.
Gym #8 - Giovanni (Viridian City, Ground-type): Like Blaine, Surf works wonders here, though also like Blaine a full sweep will need healing and crits are troubling for a Hyper Cutter Kingler. (The Rhyhorn are completely safe though. They'll be knocked off their feet before they can even act.)
Rival (Route 22, pre-Elite Four): Pidgeot, Growlithe and the starter are handled the same as in the previous fight. Alakazam now has Psychic, and that's nothing Kingler wants to deal with. Exeggcute can only attack with Solarbeam; you can smack it around with Return while it's charging but bail before it hits. Gyarados has lost its scary Dragon Rage, though it be best to avoid it anyways; Hydro Pump spam may kill Kingler first, particularly if the latter is affected by Intimidate. Rhyhorn's easily swamped by Surf.
Elite Four Lorelei (Indigo Plateau, Ice-type): Return (or Rock Tomb for everyone except Slowbro) is the way to go here. Kingler can endure the worst attacks from everything here, though without healing it won't be able to handle all five. In particular, exercise caution around Lapras and (for males) Jynx as their respective Confuse Ray and Attract attacks could throw Kingler off its game faster than anticipated.
Elite Four Bruno (Indigo Plateau, Fighting-type): Both Onix will be swept away by Surf. Hitmonchan (who is strangely found of using Rock Tomb) should be handled with Surfs to avoid eating a potentially deadly Counter. Hitmonlee's Defense is low and can be dispatched by Returns, though it's best not to face it unless Kingler's health is full/near full as it also hits very hard. Avoid Machamp; Bulk Up + Cross Chop is too much even for Kingler's substantial Defense.
Elite Four Agatha (Indigo Plateau, Poison-type): You'll definitely want something more effective than Kingler for the lead Gengar, or Double Team + Confuse Ray + Toxic will have you tearing your hair out in sheer frustration. Return/Rock Tomb crushes Golbat (barring Confuse Ray being overly haxy). Haunter will half-kill itself with Curse and one Surf will finish the job. Mud Shot/Dig or Return will do for the Arbok, though Shell Armor Kinglers at less than near-full health will want to be wary of its Screech/Sludge Bomb combo (Hyper Cutters should kill before it can use that combo.) The second Gengar won't do much with its physical movepool, though don't fight it if Kingler's Defense is still lowered.
Elite Four Lance (Indigo Plateau, Dragon-type): Avoid Gyarados; with Dragon Rage and Hyper Beam it's too much for the crab. The pair of Dragonair can both be Returned to death, though Hyper Cutters should be aware that a crit Outrage could kill Kingler. That alone should give you an idea of Kingler's very poor odds against Dragonite. (Unless you have Ice Beam for some odd reason, and even then you'd have to outspeed.) Aerodactyl should be two-shotted by Surf.
Champion Rival (Indigo Plateau): Pidgeot's more annoying, having relearned Sand-Attack but is not much of a direct threat. Alakazam's still far too strong for the likes of Kingler. Rhydon's a Surf magnet. Gyarados is still a no-go. Exeggutor has Giga Drain, 'nuf said. Arcanine will kill if it uses nothing but Flamethrower, though if it doesn't Surfs will do it in. Charizard has traded up to Fire Blast; battle with Rock Tomb or not at all. Blastoise is as bulky as ever and now has Hydro Pump, so steer clear. Venusaur only has Solarbeam to attack with; hit-and-run is not recommended though, as that will only give it free setup turns.
Post-Game: Mewtwo's way more than Kingler can handle and much of the league rematches are off-limits due to them wising up movepool-wise. Just retire the crab already; it's done enough.
The lowest level Krabby can be caught at is level 5, at which point it will have Bubble and Leer. At level 12 it learns ViceGrip, but you likely have better moves to teach it via TM by now. At level 16 it learns Harden but its Defense is already substatial and its health is to low to be messing around with stats. At level 23 it learns Mud Shot which you might want to hang onto, for lack of better options. At level 27 it learns Stomp; once again better moves are already available. At level 38 it learns Guillotine; shame about that terrible accuracy. At level 42 it learns Protect, but that just give the opponent time to buff. At level 57 it learns Crabhammer but seeing as this is pre-Gen 4 Surf is better. Finally at level 66 it learns... Flail. How disappointing.
TMs to the rescue! ...Mildly. Return is an obvious choice for a Pokemon as built around Attack as Kingler. If you'd rather save Return for someone more worthy, Strength is a fitting substitute. (Or possibly Secret Power, if you're willing to sacrifice some power for status effects.) Dig exists as a slight power-up to Mud Shot, though given Mud Shot's secondary effect and lack of charge it's probably preferable to Dig. Rock Tomb, when super-effective, is only barely weaker than max Return and Kingler will appreciate its opponent being Speed debuffed. Surf's STAB-boosted BP is enough to be effective in certain situations even with such poor Special Attack behind it. Yes, Ice Beam and Blizzard are technically options, but Kingler really doesn't have the stats to back them up.
Unless you want to play around with gimmicks like Mimic and Substitute (ill advised, given Kingler's low health), Kingler doesn't get good options through Move Tutors. Well, during the main game at least. A curse upon whoever put Swords Dance in the post-game!
Recommended moveset: Surf, Return/Strength, Mud Shot/Dig, Rock Tomb
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.
Krabby Line Ratings
What Nature do I want? If it has to lose from any stat, Special Attack's probably your best bet, as it'll generally only be using it on opponents that already have poor Special Defense. Adamant is especially welcome; you can never have too much Attack. Jolly's nice too, bringing its Speed from "iffy" to "alrightish". Just try to avoid dropping Special Defense, as it already suffers enough in that department.
Which Ability do I want? Hyper Cutter's nothing to sneeze at (especially with all the Intimidaters floating around), but considering what a common killer crits are, Shell Armor is preferable.
At what point in the game should I be evolved? Given when it becomes available, it'll have to be evolved before getting into any major battles.
How good is the Krabby line in a Nuzlocke? A decent physical spammer but a poor Water type. It's quite likely you'll have someone to fill its role more effectively.