How does turning into water make you move faster in water? I'd like to see some manga feats.
That would be like saying air moves faster in air which wouldn't work because your density is equal to the density of the surrounding environment, making your movement speed difficult. Naturally, moving fast in an air or water environment requires propulsion and an aerodynamic structure with a mass and density large enough to easily break through the environment's density.
Kisame wins by a large margin in shark form unless, again, Suigetsu had actual water movement feats in the manga.