I bought this Black & Decker 40 amp charger in Bocas Dev Toro Panama for $20.00 less. Purchased it for emergency use.
A friend's charger went out when he was on the Rio Dulce and I lent him my B&W charger until his charger was repaired.
He's got 8 - 6 volt golf cart batteries in the house bank and used the darn thing to keep his boat's batteries (Hardin 44) up while he was living on it hanging out in a marina during hurricane season. He ran that charger for 4 months in the 20 amp mode without a problem.
If you're looking for something that is for "right now something basic" you might think about getting one to keep the batteries topped off. It's made for RV's so the boat isn't much different. And,,, you can pull it out and charge your car or a friends vessel. Not nautical but the darn thing works.