What Blueberry said. They don't cool me once they're waterlogged, but I've tried taking them off too in 90°+ humid weather, and it isn't any cooler without. They still keep the sun off.

By the time I'm hot enough to want to splash water on myself, the water in my vest is somewhere between body and air temperature (and there's not too much difference between the two on those days when I really want to ...). So that doesn't do any good. If my route takes me by a water fountain or public restroom, then putting cooler water on them feels good, just the same as soaking the rest of my clothes (and my hair) with cool water.