Matt,
I keep mine plugged in pretty much all the time. Even if fishing trips are weeks/months apart. I live in MD so our fishing season pretty much ends in late Nov. until early March. Anyway, my charger is hot while it's charging but cools off somewhat during the "float" mode. It still is warm during the float mode, but is not hot to the touch. I have never had a problem with my current charger over charging batteries. Some other models do have problems with overcharging. If you have hatches on your boat which the batteries are covered I would leave them cracked a bit for ventilation. I keep my boat in a garage so it's much easier for me to do. But if you keep yours outside, I would keep the hatch cracked open a bit and put in a few mothballs in a bowl or something to keep unwanted critters from roaming into your boat's hatches. Just a thought, hope this helps.