Magnetic sensors are sealed, so weatherproof. The central control unit should be placed in a dry spot inside. The siren alarms are weatherproof, just be sure to make the wire connections weatherproof. Mount the keypad just inside the
companionway, with a thirty second delay, enough to disarm after unlocking the
hatch.
Alarm systems are a good addition to any
boat, cruising or not. Notice how I didn't reveal anything about my own
boat? It doesn't pay to advertise.