Which water pokemon and moveset is best against platinum elite four ?
Floatzel, Gastrodon, Gyarados or any other post-national dex pokemon?
You cannot get Crawduant or wailren before the national pokedex. Empoleon is not as good as torterra starter wise. Floatzel is great becuase it can out speed Cynthia's Garchomp and kill it with Ice beam/punch. Gyrados is cool but not very good for this elite for becuase Garchomp can out speed it before it can Ice fang it.
Well, GREAT is Empoleon (Aaron, Bertha, Flint)
Other good options are:
Earthquake / Flash Cannon
The first time I went through the Elite Four, I had a Golduck. It has
Surf-Good vs Bertha and Flint
Ice Beam-Godd vs Bertha, Aaron, and Cynthia's Garchomp
Hydro Pump-same as surf
Zen Headbutt-generally a good move