I gave it Splat Bomb because it could help it with poking and it overall a pretty good sub weapon. Wave Breaker would be AMAZING for the Slosher because it combos perfectly with the Slosher's 70 damage hit. It's location effect would help the Slosher aim better while flinging ink over things like walls, and the Wave Breaker could also be used to tank a bit of damage if you are fighting in the open, something the Slosher struggles with.
Please tell me your thoughts in the comments.