jremington:
In that case, the problem is trivial.Use a cheap pressure activated switch, for example, to cut off all power as soon as the box is submerged, and turn it back on when the box pops up on the surface. The box must handle at least 5 atmospheres without leaking water.
thanks,
however, I don't think that one will work underwater 
it's hard trying to find one that will be able to go underwater even if it's only checking for surface pressure.