Problem with high voltage in a project
So in general the way I do this is that I have a circuit which looks something like this:



So essentially, the idea is that there is an input power mosfet which controls the flow of power to your circuit.  You push a button, which causes the gate voltage to rise, and power flows to your circuit. "VDD" in this diagram is the battery positive rail.  Ground is assumed to be available and shared all over this circuit.

The "small gap" is a physical one.  If water connects ground and the wire coming off of that op-amp, then the op-amp will output low, and you'll suddenly lose power.

Now this circuit needs a lot of things, it's only meant to be the beginnings of a circuit that you could use.  But it should push you in the right general direction.  The robotics team I was on built an underwater vehicle and I can tell you for a fact that the idea behind this circuit is tried and true... the schematic I have for that part of the craft though was a bit more complex because it was dealing with a lot of other parts, so you get the simplified but incomplete version for now!