The nightmare of every boater, who doesn't use his or her boat regularly. There are a couple of tips for you to help get these things to last a little longer.

1. Keep a trickle charger on them even during the summer months. The last thing you want is to be leaving for a weekend outing and having a dead battery.
2. Have two batteries installed on your boat if you venture far from shore.
3. Do install a battery switch, and mark the cables and posts clearly positive (red) and negative (green.)
4. Do not install a brand new battery and connect it in parallel with an old battery. There is a risk of fire, explosion, or at the least the old battery will drain the new one, and cut down it's useful life by about 3/4.
5. Do Not install a high energy starting battery in parallel with a deep cycle battery! They are different types of batteries and one will drain the other, leaving you with two dead batteries eventually!
6. If your batteries are acting up and have trouble sometimes holding a charge, check the connections, charging system and electrical systems. If those check out, toss the batteries (two years old or more) and get new ones. You don't want to be stuck in an emergency situation with battery problems.
7. Install your batteries above the water line. If you take on water during a storm or other situation, you may loose battery power for your radios, if they are submerged.