I haven't got the car to hit the water, but one time I tried the glitch in story mode when the car was fired back, I went through the windscreen, cleared the freeway and landed in the waterWhich was handy because normally it kills you.
Seeming there is a thread for this I will post in it.
God, the damage is HILARIOUS, I do it in multiplayer and at the moment the best car was the car that looks like a lambo/zonda. It went very close to the water and I made a sweet discovery. I pull the trigger and the car doesnt move but the engine makes a rev. I thought, this is odd, the engine isnt dead. Two of the wheel were jammed and the car was trash. I did a big handbrake rev and then let go of the handbrake and it moved about half a metre. I had to keep revving it to get it to move just half a metre, quite epic actually. I gave up after about 2 metres.