Buying a light is always a good thing in these halls
But specific mission like diving require specifically designed lights, and I imagine a failure is not a pleasant event under water.
IIRC the "classic" dive light manufacturers are UK and Pelican. Usually the typical flashlight is not your best friend under water. Most of the high end lights will work, but you never know the risks of failure and possible weaknesses of the designs when they're used in a context they aren't built for.
I would trust that light in just about any circumstance other than diving, lots of reports around here of fenix's working after an accidental trip in the washing machine.
Get yourself eight (or 16) AA Eneloops and a Princeton Tec Surge. The light is a chunk, but it's got about 115 focused lumens. It's not as bright as the P2D, but probably throws better due to the focus.
I would second any of the Princeton Tec lights. They are relatively inexpensive and truly dive rated. But, I do wonder if sealing a P3D in a plastic bag (or condom) would work... Obviously, that voids the warranty on the condom.
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!