If you are going the alarm route...go with BoatAlarm.com / Flagship Marine Security. http://www.boatalarm.com/
They have an alarm system with strain gauges you bond to the deck/floor. As soon as someone steps on the floor, the minute flexing of the floor activates the alarm. Very slick, and no false alarms. It will NOT go off unless someone steps on the deck or opens a hatch.