You were most likely advised by the boat yard to remove the batteries, although I did not read where you mentioned this. If they are still onboard, subjected to very low Canadian winter temps, your boat may be at risk of damage.
An uncharged battery's electrolyte is more susceptable to freezing than one fully charged. The expanding fluid could crack the casing, leaking corrosive acid into the bilge.
True Blue . . .
sold the Nauticat