For my car, I use a Wal-Mart 1.5amp battery charger/maintainer. I use the cigarette lighter socket to make the connection to the battery--I wired it to a cigarette lighter plug.
I think that's much safer (and easier) than making a connection directly to the battery.
Of course, this will only work if the car has a cigarette lighter that is live when the key is off..and do boats even have cigarette lighter sockets?
