Cloyster for Shell Smash
Kabutops for Swift Swim ability
Jellicent for special Walling
Jolteon for Rain Dance and taking T-Bolts with Volt Absorb
Holy water types, Batman!... though I guess that makes sense on a rain team. xd It would appear you've decided upon Jellicent, though. It's decent enough for ignoring any fighting attacks that could be coming in the direction of Cloyster or Kabutops... but Tentacruel resists fighting, while providing rapid spin support for Cloyster.
A tough one, for sure. I'd need to see the other two teammates to decide on that one.