Cal is correct. Proximity on a switch, mag switches to rear cargo lids, spotters seat, what ever compartment the battery is in and anything else you want covered. We have done a few with tower light flash hooked up also. Adding additional or louder sirens or an air horn is a plus too. One thing to keep in mind is current draw. Although the current draw of an alarm is very minor it can be a problem if the boat sits for a long time without a battery tender being connected.
One thing that is hot is tracking. Lojack can work on anything that you want it to but it is not as affective as the C3. The drawback with Lojack is only that it is only activated after you report the boat missing. This doesn't do much good if you are away from home for hours at a time or away from where the boat is kept for days to months at a time. Once it is activated the LoJack emits a RF signal that is only received if in range of a tower or police car that happens to have the equipment to detect it.
What you really want is this: http://www.c3autopageusa.com/
The C3 can alert as many phones as you want to pair with it when the boat is tampered with or moved. C3 operates off the 3G network in addition to a GPS antenna that can be mounted anywhere in the boat or car. C3's primary use is for vehicles but nothing is stopping us from using it in boats. Just about anything is possible with the unit. If you sum it up the C3 is a security system with unlimited range, GPS tracking.
Let me know if any of you want me to send you the demo software to your phone.