You can leave the batteries in the boat, but i would put a charge to them every month for an hour or so...Nothing higher than 10-20 amps though...No need to boil them...Some say it is easier to pull the batteries and store in the garage (not on the cement, they'll discharge).
My old 1 ton 4x4 sit's out back and the battery is okay in that, but from not using the truck all the time the battery life is cut in half from being idle.