You got your boat out of the water before ice-up and winterized the engine, but don’t forget that starter and deep-cycle batteries used for things like trolling motors need special attention to prepare them for winter. They are expensive, and many boats have more than one.
No matter how you store your batteries, start with a cleanup. Grease and dirt can accelerate their discharge, so get rid of that stuff with water and dish soap. Remove acid and corrosion with battery cleaner or a paste of baking soda and water. Clean terminals with a wire brush, and rinse away the works with water.
Freezing usually means battery death. A fully charged battery can withstand freezing down to about -60°C. But a discharged battery can freeze at -10°C. Pretty obvious, right? Make sure they are fully charged before storage. For “flooded” lead-acid batteries (the kind with removable caps), top up the acid reservoir with distilled or deionized water before charging to allow them to receive a full charge. The liquid should be at least 1/2″ above the lead plates and no more than 1/8″ below the fill tubes.
If you store your batteries outdoors in a boat, disconnect the main electrical leads to prevent discharge. Done. If you can store batteries at room temperature, it will be easier on them, but not strictly necessary. Inside or out, if you can charge your batteries periodically over the winter, it’s a bonus. Fun fact: there is an out-of-date concept that batteries stored on a concrete floor will have all their electrical mojo sucked out of them. It’s not true.