So, I would posit this issue with the defense system: If your thing is underwater, than the chances that the intruder can survive at that depth are pretty high... It's not that you couldn't have a guy in scuba gear, but given the amount of Atlanteans I've seen from browsing the sight, it's not like it's uncommon... You might want to add some non-environmental based defenses. Maybe include an open circuit system that is high off the ground... this will create no power issues under normal operation, but when the compartment is flooded, it closes the circuit and shocks the intruders (Water and Electricity == bad, or Things I've learned from Pokemon)... should cover your people who survive the first round drowning? Include something about low amp discharge, so you don't kill them... but you're first defense involves drowning, so... yeah...
