On lead acid batts, POWERSTREAM sells dc converters. Email PS for advice. PS sold one here for 12V to Marntz PMD 620 voltage then phantom power to a Mineroff hydrophone. An Attwood 12V bilge pump swallows 12V.
Add a Pelican box with plastic thru wall fittings from US PLASTICS or McMaster Carr.
Carrying 2 Curts should clear the cloudy day