If you have a good moveset for Wishiwashi, post an answer below and upvote the best ones. Remember, this is for competitive movesets, not in-game. Ability, EVs etc should be included, and we encourage sets for VGC doubles as well as singles. Make sure to read all the guidelines here.
Wishiwashi Pokedex & learnset for reference.
Wishiwashi @ Leftovers
Type : Water
Ability : Schooling
Nature : Quiet ( Spa+ Spe- )
EVs : 128 Def / 252 Spa / 128 SpD
Wishiwashi is really great while Schooling. The strategy is to allow him to sweep while Schooling, and use Aqua Ring to remain at high Hp. It would be great if you pair him with a guy that knows Trick Room. Like this, you would be able to start first. Aqua Ring combined with Leftovers give him lots of Hp, allowing him to stay schooling longer. Surf is STAB and is useful for Doubles and sometimes singles. Hydro Pump and Scald are also STAB that work great in both Singles and Doubles. Ice Beam is there to deal with mons that resist its STAB moves. U-Turn is there to allow him to switch out to prevent from being set up by Water types or some other mons.
Wishiwashi @ Shell Bell
Nature: Quiet (+SpA, -Spe)
EVs 232 HP / 236 SpA / 40 Def
The damage he does compared to his relatively low health stat means Shell Bell heals more than a Leftovers would. Use Aqua Ring on the first turn for passive healing for the duration of the fight, Wishiwashi always goes second so additional healing is nice. Main damage will be coming from Scald, which has good power, is STAB, and has potential to leave opponent burned. Ice Beam and Earthquake are for situations where the opponent is either weak to one of them or resistant to water attacks.
WishiWashi @ Leftovers
EVs: 252 SpA / 176 Hp / 80 SpD
The idea is to keep your HP up to stay in Schooling Forme, so Aqua Ring and Leftovers help with that. Protect helps stall to recover HP. Scald is an appropriate STAB, because it's nice if the opponent has a burn while stalling. Ice Beam is just a nice, strong, accurate coverage move that covers half of your weakness.
Hope I helped.
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EVs:252 hp 252 atk 4 sp atk
Wishiwashi @ Leftovers
Nature: Modest(Spa+ Att-)
EVs: 252 Hp, 252 Speed, 6 Special Attack
Scald(stab and a chance to burn for less physical damage and extra damage)
Ice Beam(coverage against grass types)
Aqua Ring(for healing and staying in school form)
Brine(for when you're almost out of school form)
You could also switch out brine for U-turn for if you need to get out of a bad situation.