I've often wondered what the difference is or what the assumed risk is in using a light under water. For example, my Inova XO...I turned it on after buying it, put it in a sink of water just a couple inches beneath the surface, and let it run for about an hour. After that hour, I opened it up and noticed some water got in the battery compartment, and some condensation was inside the lense. I dried it out good and after some normal use, the condensation under the lense dried too...but left a smudged lense from the mineral crap in the water...
So what I've come up with on lights, like the P3D which I have (and won't be testing with) is that they're "dunkable." If I jumped into a swimming pool with it at night to look for something that was at the bottom, I might be ok. My guess is that it's simply water pressure that is the difference between something that's dive worthy, and the simple o-rings on most lights. Those O rings and the precise machining of the light itself are probably what make or break under water pressure.
That being said, I'd recommend for the OP one of
these babies!