Bad ground on a boat can happen when switches or wiring become worn or come into contact with water. In order to test and find this bad ground, you’ll want to switch everything off, but keep the battery switched ON. With your multimeter set to DC, take a voltage reading to determine if there is a leak. If there is, depending if the leak is on the boat side or the battery side, you will need to rewire the battery or fix the issue in your boat’s electrical panel. Most leaks, however, should be noticeable to the naked eye and solved with a quick fix.