We keep the usual Coast Guard required items aboard, including a cell phone. One thing I might add is to store the items in a waterproof bag that floats.
The wife thinks we should tote an inflatable. I think it's good idea. It gives her a way to get home when she asks silly little questions, like "do boats come with names on them?"
The battery idea was good. It's been awhile since we've checked them. Thanx!