> And of course for melees, an enemy must have the opportunity to cut the pipe somehow, otherwise underwater camping will be possible.

What about the obvious choices: destroying the pump and/or the power supply? Someone in deep sea is most likely unable to defend the base.

I don't even see the point in under water camping but whatever ;) say a direct explosion hit on the pipe end will destory it. Anything like a low level attack ("cut the pipe whereever you want using a sword") will make the whole thing useless (and of course it's virtually impossible to script right now since lines do not interact with any object).