Save your back and leave them on the boat. Only a discharged battery will freeze. If your batteries are in good shape charge them fully before storage and either turn them off or disconnect them if you feel you might have some drain. If possible come back once during the winter and plug your boat in to give then a charge for an hour or so. Don''t forget to disconnect the bilge
pump, they are sometimes wired directly to the battery in Catalinas. I used to remove my batteries until I saw a couple of well respected yards store peoples batteries in an unheated shed.