Well, if you only want 1 battery, the biggest 12v you can get is 54#. Get the Maxum Miser model. It will help with battery life a little. I would find some way to go 24v and the most poundage you can get. Bigger is always better in an electric.
Yeah I am his son, I will tell him you said hello. I also agree with BobT can't have enough thrust, especially in the wind when you need it. You could always run a cable to the front, but can you get under the the floor in that boat?