Depends on the boat ramp.
Probably not a good idea if the water is salty, for obvious reasons, or the ramp has a drop off the end, drop a wheel over that and you might struggle to get back up and sink the vehicle.
Getting water in the clutch housing can let the clutch slip horribly.
Fills the brake cables with water.
I wouldn't think it washes off that much, high pressure water from a hose would do a better job. Try caking your arm with clay and dipping it in water, you need movement or rubbing to wash the mud off, otherwise it just gets wet.
Personally, I'd think it would create more maintenance issues than it solves.