The double o-ring seals on the TailCap ends of many SureFires allow you to rotate the TailCap underwater. I guess that as long as all other parts of the A2 are watertight, the TailCap o-ring seal will be good as long as you don't rotate it underwater.
I took a modified L1 down to 20' and there was no water intrusion through the tail switch or battery tube to head O-ring seals. I did use both the momentary as well as constant on feature of the switch. Just one data point but none the less, a point on the "good" side! Although not an A2, the switch is the same as well as similarities in the battery to tube O-ring seal.