Toxapex @ Black Sludge
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Scald
- Recover
- Haze
- Toxic Spikes
Toxapex is your main option for a bulky water. With 50/152/142 Bulk, Water/Poison typing, and Regenerator, Pex can be a great defensive Water-Type for you.
Other niche options:
Slowbro @ Leftovers
Ability: Regenerator
EVs: 248 HP / 96 Def / 164 SpD
Calm Nature
- Scald
- Slack Off
- Ice Beam
- Flamethrower
Slowbro is more of a pivot, but it can be a nice check for a few threats if you struggle with something like Medicham or Keldeo.
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Scald
- Recover
- Earthquake
- Toxic
Water/Ground gives Gastrodon a small niche for walling a few specific targets and that's mostly it.
Overall, I'd say try Toxapex first. It could be really great for you. The others mentioned are more specialized so only use them if you really need what they provide.