If your boat has a gasoline fuel tank mounted below decks, you should have a vapor detector. Also known as "fume sniffers," vapor detectors monitor for flammable gases such as gasoline fumes.
Vapor detectors are mounted in the engine-space bilge, just above the slosh height of bilge water, with the sensor away from the hottest parts of the engine, such as manifolds. Vapor detectors are almost always hard-wired to the boat's 12-volt DC system. Usually, the unit has a control head mounted at the helm that will sound when dangerous fumes are detected in the bilge. The wire that connects the sensor to the head unit typically can't be cut because the manufacturer has calibrated its length.
Some vapor detectors can turn on the bilge blower when they detect a buildup, a smart option. The blower, of course, must be ignition-protected. Look for an alarm that is UL 2034 listed.
Vapor alarms should be tested monthly using the manufacturer's procedure. You can also test the sensor using a butane lighter by depressing the lever lightly to release a small amount of butane gas next to the sensor.
Replace vapor detectors after no more than five years, or right away if they become submerged.
If the alarm sounds at the fuel dock, it may mean that a large quantity of spilled gas is in the bilge; it should be dealt with by pros. Make sure everyone is off the boat (as they should be while refueling) and call 911. Don't operate anything electrical, including the blower; it won't eliminate spilled gas. If it sounds while you're underway, call for help on a handheld VHF (less chance of sparking than an installed one) or a cellphone.