Marine batteries
If the batteries are stored in a garage, that should be OK. IIRC, its a good idea to keep them fully charged over the winter. An inexpensive "smart" charger like a Vector (cheap at Wally World) will do the trick. Hook it up, plug it in and forget it until its time to take the batteries back out to the boat.Another option if they're inside a warm house is to charge them fully every couple of weeks to keep them fully charged, then charge them completely a day or two before taking them back out to the boat.If you're going to use one charger on two batteries, use it on individual batteries one at a time.As far as keeping them on the boat, if a car battery will freeze (how often does that happen?), then a marine battery will freeze. A fully charged battery will survive the winter better than a discharged one. If on the boat, its better to keep a "smart" trickle charger hooked up.I keep my battery in the garage (cold but usually above freezing) with a smart charger hooked up and on all the time.