How old are the batteries ? Reading 12 volts doesn't mean any thing ,it's the amperage that you have to go by. You could wire two 9 volt radio batteries together and get 18 volts, wouldn't have enough amps to turn over a starter motor. If the batteries are dead they wont hold a charge and jumping them might work IF you had excellent connections, someone revving the car up and you waited a couple minutes before you gave it a try while the car is still revving up. You could also take the car battery out and install it in the boat and see what happens. Its probably that or your starter is shot.