Try and fill up the tank at the end of the season to help avoid this problem. Condensation also produces water in the engine oil and oil filters. This problem increases when the engine has been periodically warmed up during the boat's lay up, and allowed to cool down during a chilly evening. Even with limited use, change oil and filters each season. It is not unknown for a neglected oil filter to rust through and leak out all oil, resulting in the seizure of the engine.
Water is also an enemy of the boat's electrical system, and in particular the batteries.
Keep the terminals clean and well-protected with vaseline. Don't use grease as this acts as an insulator as do all oil-based materials. Using an electrical spray will prevent corrosion and bad contacts. Concentrate on the starter motor and alternator contacts as these have to handle heavy currents, and poor contacts here can produce serious consequences. The engine's cooling system is another item very often neglected.