This guy might be able to set up with his Focus Sash provided the opponent doesn't carry priority moves. Aqua Jet, combined with Adaptability, makes for a priority-80-Base power water attack. Aqua Tail also doubles, dropping in at 180 base power. With no defenses but a sash to keep you in the mix to set up speed, Agility will double up your speed to ensure your choice of Flail or other moves might hit first. I personally haven't used it but I intend to breed such a fish.
Basculin @ (Focus Sash)

Trait: Adaptibility
EVs: 252 Atk / 252 Spd / 4 HP
IV's: Try for high attack and even speed if you can, "Fantastic" or "Can't be better" from the IV checker will be what it needs
Adamant Nature (+Atk, -SAtk)
- Aqua Jet
- Aqua Tail
- Flail
- Agility