Most of the pokemon people have mentioned all have great potential such as Starmie, Milotic, Ludicolo, Kingdra, Gyarados and Swampert. I prefer swampert though due t its great dual typing and resistances. Although it does have a horrible 4x weakness to grass but thats avoidable enough by switching. If you would prefer to use one of the others people have suggested go ahead becasue its all up to your opinion and the other choices have great potential also.