Even free I doubt they would be worth it as they take up too much space. Maybe get enough to keep batteries charged while away from the boat. Determine that by checking voltage every time you visit the boat (you already do that) and keep adding until you find the voltage does not drop between visits but is not higher than about 13volts, or get a regulator at that point.
Have faith that the oceans are going to rise and flood the world, that plague and pestilence brought on by Climate Change is going to punish us for not believing. Please do as they say it is our only hope. :P