You want to run those batteries down to zero?
When you have run the boat for a day and the batteries are low, park your truck close to the batteries. Take the positive wire off the truck battery. Hook a set of jumper cables to the boat batteries. Neg. cable to the truck battery. Positive cable to the battery wire. Turn your headlights on bright and leave it for a couple of hours or until the lights go out. Rehook the battery wire and your back in business.