These subs routinely dive to 20-30 ft and possibly more. Propulsion is from bilge pumps which are apparently sealed very well. Electrics are well sealed with only wire protruding.
http://www.youtube.com/watch?v=ntKT1araF3U
http://www.youtube.com/watch?v=pAZyL...e=user&search=
But I don't know crush depth?