One of my batteries was showing 10.2 V.Is it revivable?I've put it on charge but my hopes aren't high.The reason for this is that I have no means of keeping the batteries topped and haven't been able to use the boat.
The battery will charge up but only up to 70% (assuming it is lead acid battery), depending on the quality and how much damage has been done and provided that you don't have a dead cell, it should hold its charge and may carry on for years; however during the cold months you will notice the difference. Nowadays, solar panels are very efficient and low cost; you can get a 20 or 30 watt solar panel, as a trickle charger, to keep your batteries charged up. Try charging your flat battery with ta three stage charger initially.