I wouldn't bother with DC on that setup. You aren't running the motor long enough. Multi stage charge at home to keep them tip top and the motor will do the rest on the water assuming you don't have voltage drop to the batteries. I wouldn't run a VSR either, just the old rotary dial manual isolator. Tried and tested and simple which is usually better in a marine environment. Unless you are running solar and banks of batteries a complex management system isn't really required.