Batteries must be kept clean and dry. If the top surface of the battery gets dirty or wet, a small flow of electricity will flow between the terminals as the water creates a conductive path. If installed in a location where water can drip onto them, this can cause the batteries to discharge fairly rapidly. When boats bounce around, the electrolyte fluid in lead-acid batteries will spill out through the tops. This creates an excellent electrical path for stray current and can lead to rapid discharge.
Clean battery tops with a damp sponge or rag, then wipe dry with a paper towel.
This should be done at least biannually. Batteries must not be sitting on a wet or damp surface. To understand the importance of this, just take a fully charged battery and set it on an apparently dry concrete floor. It will discharge right through the casing in about a week. Even the plastic casing is not a total isolator.