Water on top of a battery is from "boiling" the batteries, aka overcharging, or charging at too high a rate. The best cure for this is to use a so-called "smart" charger which usually has three stages. First stage is a high rate of charge, then as the battery nears full charge, the charger drops to a medium rate of charge. Finally, when the battery is fully charged, the charger drops to a "float" charge which holds the battery at full charge without overcharging it.
The advantage of having the charger on all the time is that if the boat should start to take on water for some reason, the charger will allow the bilge pump to continue running until--- or hopefully until--- someone checks on the boat. If the charger is off and the bilge pump starts cycling to take care of a leak, the pump could run the battery down at which point nothing will be evacuating the water out of the boat.
When we bought out boat it still had its original 1973 charger. This was a multi-selection charger--- you could manually select the rate of charge--- but it would not change rates on its own. Even at the lowest setting it was "boiling" the batteries, the pooling of water you have noticed on your boat. So we changed it out for an inverter/smart charger and have had no water on top of the batteries since.
