You probably want Walrein, honestly. It learns some pretty good moves, and there are a lot of types Ice and Water hit super effectively together. The annoying part about Walrein is how it's obtained, but you already have one. 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
- Blizzard / Ice Beam
- Signal Beam / Super Fang / really anything
Hope I helped!