Walrein (STAB on Ice Beam, Earthquake)
Lapras (STAB on Ice Beam; can BoltBeam; believe it has access to Hypnosis as well)
Tentacruel (Poison is SEfx on Grass)
Relicanth
My personal opinion would be:
SWAMPERT @ Leftovers
Substitute
Earthquake
Counter/Curse
Focus Punch/Brick Break/Return/Body Slam/Strength (if you are not using an HM slave)
LANTURN @ Leftovers/Chesto Berry, Volt Absorb, Modest (EV trained in SpAtk)
Thunderbolt
Surf
Thunder Wave/Toxic/Confuse Ray/Protect
Rest
STARMIE @ Leftovers, Natural Cure, Timid
Thunderbolt
Surf
Ice Beam/Psychic
Recover
(Psychic is a viable option if you would rather save your Ice Beam and have some Fighting/Gengar coverage.)
LUDICOLO @ Leftovers, Modest or Calm
Leech Seed
Giga Drain/Synthesis
Toxic/Rain Dance/Synthesis
Surf
(Obviously, if Rain Dance, then not Synthesis. You can also be incredibly annoying with Rain Dance, Rain Dish, Leech Seed, and Double Team. Just sayin'.)
One of the following:
SHARPEDO @ Salac Berry if you can get it, Timid nature, maxed Speed EVs
Endure
Flail
Crunch
Surf
RELICANTH @ Choice Band/Leftovers, Rock Head, Adamant nature
Double Edge/Sleep Talk
Rock Slide/Sleep Talk
Earthquake/Sleep Talk
Rest/Yawn
And one of the following:
LAPRAS @ Leftovers, Water Absorb, Bold or Modest
Rest
Sleep Talk/Surf
Thunderbolt
Ice Beam
WALREIN @ Leftovers
Surf
Earthquake
Rest
Sleep Talk/Snore
(Plays like poor man's Swampert, only not quite as good.)
or
MILOTIC @ Leftovers, Bold
Surf
Ice Beam
Recover
Hypnosis
(Switch into Thunder Wave or Will-o-Wisp and watch as Marvel Scale makes Milotic a very unkillable Loch Ness Monster.)