You probably want Walrein. It learns some pretty good moves, and there are a lot of types Ice and Water hit super effectively together. The biggest downside is how tedious it can be to obtain, but since you already have one, that’s not an issue. Thick Fat is the better Ability — if you can, get one with that. While it doesn't get a lot, it functions as a great HM slave or mixed attacker.
Jellicent is also quite good, but it's a bit less bulky. Will-O-Wisp and Recover are never really necessary in-game, and it doesn't have many other options.
Walrein @ Leftovers
Ability: Ice Body / Thick Fat
- Surf
- Blizzard / Ice Beam
- Strength
- Signal Beam / Super Fang / really anything
Hope I helped!