What is the strongest water Pokemon non legendary?

It depends on how you look at the Pokemon stats. Plus, if you think your favorite water Pokemon is the strongest, then so be it.

Everyone has their own opinions.

By total base stats its a tie between Gyrados, kingdra, and milotic, they all have a total base stat of 540. But their individual stats arent exactly the same just overall its the same. Gyrados has the best attack among the three, and ties with milotic for the best HP. Just look up list of Pokemon by stats, and go to serebii.net and it shows all Pokemon just look for the types on the list