I've got a new 25hp 4 stroke mercury and I'm wondering if I need to do some kind of preventative maintenance before this cold snap?

Trim it all the way down and drain the water out of the lower unit.

Pull the drain plug out of the gearcase and check for water. If water is present, drain lube/water and refill with fresh lube until you can get the leak fixed. If lube is good, reinstall plug. Water is heavier therefore it will be the first thing that comes out of the drain plug if water is present.

Thanks