I've not "dived" with SureFires - I've used an M3T and M2 in the sea snorkling.
M6, M4, M3T, M2, G2Z, 12PM in a swimming pool dive pit (it's about 4 metres deep I think)
It's important that the flashlight is not subjected to a rapid change in temperature if possible - this could cause issues with the various seals.
For example, when I first visited Las Vegas I was in a hot tub with two girls using my M6 as the only illumination. We would swap back and forth between the hot tub and a cold pool with the M6 being thrown in before us. The TailCap seals failed and the M6 flooded. Leaving us in the dark. Shame.
This was back before SureFire changed the seals to make the TailCap tighter, and the M6 had a Lexan window I think this was back in 2000...
I have no concerns taking a modern SureFire 6P underwater. The G2 - if it is a current version with a LockOut TailCap I expect it'll do okay.
Do not rotate components underwater unless they have two o-rings. The Millennium Series models have two o-rings on the TailCaps.
Al