well, there should be double O-ring seals in the Dereelight lights, however I wouldn't dunk it for very long, or too deep, they're just o-rings and sustained pressure will cause water to seep past. (None of them are dive lights for example)
I kind of had the same question because of the o-ring for the lense sometimes sticks out. You have to be careful and most of the time i have to use a pencil to push the o-ring to make it fit in its rightful place. I have not yet tried using it underwater (i only got it a week ago).
But yeah, its "waterproofness" does kind of make me nervous.
My DBS V2 is sealed up better than most of my lights. I wouldn't take it diving, but it should be okay if it was submerged under a couple feet of water.
I too have had my DBS v2 for a week or so, although I haven't tested its waterproofness. The double o-rings around the head and tail should provide better than average waterproofing there. The head o-ring is a bit small, and you do have to be careful to fit it in after swapping the reflector or lens, but I don't know if this has any impact on waterproofness.