Buying a light is always a good thing in these halls :D
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.
bernie