I bring mine inside, because I have to (I live in an apt complex, and the boat is 100yds away).
But, if you can reach them with a charger/extension cord ... charging them up to full charge, at least once/twice a month, would allow you to leave them in the boat/outside. But, if there's even the slightest chance that they could freeze ... I'd bring them in and charge them up once a month.
... cp