A bag's skin is flexible, the wire thru is flexible, conditions not giving reliabilty with sealants, pitch or West epoxy. Stability from two wide fender washers holding a solid nylon vertical tube then a relatively felixible silicone wall for the wire does work. Clean wires and roughen, clean and definitely check for 'hold' or 'grip.' The hydrophone coax came loose when I stepped on it.
Hydrophone parts are available at: http://www.allelectronics.com/
There was on online schematic. I'll check on it.
Summit Racing online sells a Curt 12V 5 ah 'trailer battery.'
One trick mod would be mounting electronics down on the hull in the forward hold, operated with remote control buttons in the deck dry bag on forward combing.