The battery in your boat needs care just like everything else in your boat. When stored they should be fully charged. The electrolyte should be checked with a hydrometer before water is added. If the electrolyte is low distilled water should be added but only when the battery has been fully charged. When adding water the fill level should be about 1/8th of an inch below the top of the well and you should wear safety glasses and be careful not to overfill.
It is important to keep the battery compartment ventilated, as the batteries will put of hydrogen gas while they charge. That being said you should keep possible sources of ignition away from the battery banks. Once the level of the electrolyte has been checked the vent caps should be secured tightly. The terminal clamps, posts and battery casing should be kept clean and dry and the terminal clamps and posts should have a thin film of petroleum jelly on them to help protect them from oxidation.
If you aren’t comfortable adding water, performing other battery maintenance, or using hydrometers to test the specific gravity of your battery your marina should be able to help you.
Hope that spreads some light on it.