Why not just go to your local auto parts store and a simple cheap charger? 5A chargers seem to cost $40-100. That's what I would do.
Then again, I don't see why a Pb-capable charger wouldn't be able to charge a car/boat battery. The chargers are current limited. If you set it to 5A, it's will never try to put out more than 5A, even if you're trying to charge a battery for an 18-wheeler. As long at the voltage is right, you should be fine.
Maybe I'm missing something here, but I don't see what the risk would be.