I carry my Honda 2 kW generator with me in my boat, it isn't intended for marine use, and can pose a significant carbon monoxide risk, but in the event of a dead battery, you can use it to charge your battery quickly, just make sure to position it so that the wind carries the exhaust away from the boat. Obviously don't take a nap or go inside an enclosed space while its running.
I've never had to use it, but it gives me piece of mind.
btw, if you are running an electric start kicker, you'll be fine, it will charge your batteries. If they are ever too low to start your main, just let the kicker run a bit longer