You describe ONE of the problems I've encountered. As you've pointed out, actuating the piston drive underwater causes a small amount of water to enter the light.I think what Gnius is saying is that its a bad idea to actuate the Piston Drive underwater right? (I had suspected as much but never tested it myself) Actuating the piston causes the piston's O-ring to move which could let a small amount of water in, but this should not be a problem even if you are using it as a dive light because the SmartPD series of lights can also be turned on/off using the twisty function which will bypass that particular situation entirely. I wouldn't mind seeing Nitecore use a rubber seal over the piston button like clicky lights do for an added level of insurance though. 4sevens is coming out with their own series of lights soon called Quarks, and the specs and prices on them sound like a very good bargain.
However, what bothers me a lot more, is the (even smaller) amount of water that enters the head assembly when submerged. This is not due to the piston drive, but rather to the lack of a lip against which the lens can be secured with an o-ring. As I've mentioned before, the floating lens is a design oversight. I've sent the diagram of the problem, and the explanation of why this cannot be solved without changing the design of the light to Nitecore, and was told they'd look into it.