whenever you walk on water, your density drops to 0... so no jutsu can hit you :O

EDIT: only the contact jutsu are unable to hit on water. Area jutsu (divine strikes, konoha senpuu, Karamatsu no mai, and the big fireball Katon for example) all can hit the opponent still, as can melee strikes.

EDIT2: the density thing is best guess, we walk over virtually anything including eachother... theres also another water glitch. if you get teleported while on water, including by Shushin, you are considering as on water still even while moving around.