If they are in the boat and hooked up, make sure that you don't have anything pulling juice. Try unhooking them from everything except charger, charge them, then unhook charger. If you still are losing voltage, they need to be replaced. You don't want them to die in the middle of a trip. If they are good it is time to look for the drain. It could be something left on like ignition or light switch or a bad rectifier could cause batteries to discharge.
If yopu do need new batteries, I second the Walmart batteries. They have a good warranty and if they go out on a trip there is a Walmart on every corner. It makes it easy to get them replaced.