Well, there isn't really any ''good'' water Pokemon. Legendaries are always strong, and a great one is kyrogue, which you can get in Pokemon emerald. It really all depends on which moves the Pokemon knows
cheat
Barboach can be caught using the Good Rod in many bodies of water in Pokemon Diamond. One of the areas where it is easy to find with the Good Rod is the Great Marsh.
you need to trade one in Pokemon diamond or go to the great marsh and to
Remoraid is a pokemon(water type)
to evolve certain Pokemon
evee
swampert miltic
You can find Yanma in Pokémon Diamond in the Great Marsh.
You have to get a manaphy from Pokemon ranger and breed it
In Diamond, you can obtain a Water Stone in the underground in Sunyshore City.
in the great marsh in the great marsh
anywhere there is water