Years ago I played around with a tow boat design that used a Sevylor blowup raft motor. It's similar in size and design to a trolling motor but doesn't use as much current. The company sold them with a SLA battery for power. For simplicity, it could be wired using a servo and heavy duty switch for forward/reverse.

Here's an example:

http://www.ebay.com/itm/SEVYLOR-12-V...p2054897.l5660