It's funny this subject came up. I finished installing an alarm system on my 17' Tracker only yesterday. It has sensors on each of the hatches, a shock sensor, a floor pressure sensor, and a tilt sensor. When I'm docked on the water I can disable the shock and tilt sensors.
Total cost, $18.00.
I'm pretty savvy electrically, so I bought a car alarm off e-bay and modified it to suit my needs. I had all of the sensors (magnetic switches, etc.) on hand because I installed an alarm system in my home a number of years ago and had some leftovers.
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'Tis better to have fished, and lost, than never to have fished at all.