Use underwater for scuba diving voids your warranty on non dive depth rated lights.

If I dunk my M900 underwater it still works but the LEDs turn on low power and stay on for almost an hour till they dry out.

I've taken several different SureFires underwater with no leaks.

If you need to have to take SureFires underwater than it's best to talk to SureFire Tech Support to see whether they can advise you.
It's all about correct o-ring care.
Test out prior to needing it for real is my advice.

What models are you considering?

Al

Just don't "bump fire" it under water!  

I use my Surefire M3 as a divelight.  Its been down to 110 feet, which is around 4 atm of pressure, many times with no problem.  I don't think I'd take like a 6p or z2 down that deep, though.  

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

Quoted: . 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. Al

Wow.

When I think of water and portable lights I think PELICAN!  They are one of the best companies I have ever delt with and they have a lifetime no questions return policy.  Many of their lights are good to 500 feet depth.  The one I would want instead of my surefire is the 2410 Recoil LED it rocks.  Their cases are the best bet for protecting your black rifle too.  Very friendly staff and I will always sing their praises for the great stuff they make and their bussines practices.

The Army Owns the Night.

Very Respectfully,
David