Engine breakdowns at sea in small boats is a major cause of foundering, and another good reason to have a sea anchor on board. In a pinch, throw out your regular anchor. If you're one of those people who doesn't perform preventative maintenance, you're heading for trouble. And, of course, you're going to check over that bilge pump wiring and float switches frequently and make sure everything is in operating condition before each and every time you head out. Don't fool around with old batteries, replace at the first sign of weakness.
Many people make the mistake of replacing paired batteries one at a time.
You already know that you can't do that with dry cells because an old battery will drag down a new one. Whether you like it or not, batteries have to be replaced in pairs. Check systems over carefully. Little things like loose wires and corroded wire connections can lead to major problems. Keep in mind that a boat rocking and rolling at sea is what tends to cause things to break down.