A simple way to check would be to remove the module/LA and cells. Reassemble, Tie it to a rope and dunk it for 20 minutes however deep you want. Pull it back out, dry the outside thoroughly and inspect the inside for water leakage.
The www site says its double O-ring'd to keep out water, but theres still the forward momentary switch and front lens area. I have lights for example that are very water tight all around with tight fitting O-rings except for the front lens... or switch boot. So just because its got a bunch of O-rings around the body tube ends, does not mean its completely water tight.
If it is water tight, and its a twist switch it should NOT be twisted on or off while submerged. The vaccuum created by twisting the cap in/out and twisting/sliding motion of the O-rings can suck water into the light, past the O-rings.
IMHO you are better off getting a dive-rated light. But then agian the nice thing about P60 hosts is that they are completely re-buildable.