Batteries wont last forever and there are a number of factors that can cause issues with them. If you want to extend the life of your boats battery and make sure it works next spring, it's important to take care of it in the off-season. Upkeep and proper storage can save you a lot of trouble and give you more time on the water.
The first thing I will tell you would have to be to disconnect the terminal connections. This will eliminate any electrical loads and small power draws. Self Discharge and a slow drain can damage batteries in the off-season. After doing that, go ahead and clean any corrosion that you may see. This not only means on the terminals of the battery but also on the connectors of the cables. Keeping your terminals and connectors clean is key no matter what time of year it is.
If you are able to fill the cells of your battery, I would also suggest doing this. Remove the vent caps and fill each one with fluid to the appropriate levels.
When your ready to put the boat away for the winter, go ahead and make sure that the battery is fully charged before doing so. Whether you keep the battery on the boat or bring it inside the garage for the winter, you should be charging the battery using a trickle or automatic charger. This will stop self discharge and help extend the life of your battery. Batteries that are fully charged are also less likely to freeze. I personally like to take them out of the boat and place them in the garage on a wood surface so that I have easy access to them all winter long.
Its really not much but it can save you a huge headache when you bust the boat out first thing next spring. It will also save you from spending some of your hard earned cash on a new battery. The way gas prices are right now, I know I dont want to be throwing away money at something that could have been easliy avoided.