Mine goes in the garage on a board or foam on the floor and get's checked 1 time mid winter and then again as the spring is coming and then thrown on the charger. Why, old habit... I'm sure it would be fine in the boat, covered outside the garage but old habits die hard... This year was the 1st time in 6+ years I needed a new cranking battery following my old routine BUT there was a constant draw draining it that was found later after there was no saving it...
Less work if it's/they are safe in the boat and you keep checking on them to ensure they are not drained...