The LEAF will protect you from this, but it is a universal fact of battery charging that you can't charge a battery using less voltage than the battery is producing. If you manage to try that, the battery will discharge through your charging device, usually with disastrous results. Think of it this way: If I set a cup of water on the ground beside my above-ground swimming pool, can I siphon water from my cup into the pool? Voltage is like the height of the top of the water in the two containers. (And stored energy, i.e. kWh, is like the volume of water in the container.)
In case you are wondering why you can charge a LEAF battery using 120 volts or 240 volts, when the battery produces nearly 400 volts, that's because there is a charger inside the car which boosts the voltage (and converts it from AC to DC).
By the way, you could get an inverter that converts 12 volts DC to 120 volts AC, and plug the output of that into the normal AC plug into the car, but see my comments just above about relative sizes of the swimming pool and the cup of water. That 60w 12v DC device might indeed be able to half charge your LEAF battery if it ran continuously for 15 days, but what is its energy source? If it is the boat battery, forget it.
Come to think of it, that wouldn't work, either. Even if the onboard charger was willing to accept half an amp (and I'm sure it isn't) the car runs a liquid cooling system while charging that draws more than 200 watts. So your 60 watt source would only end up draining the battery to supply the cooling system.
End of April 2013: Traded my 2011 SL for a 2013 S with charge pkg.