Look into this SeaKey. You can set whats called geofence and if the boat leaves a preset perimeter you will receive a call from SeaKey letting you know and they can track the boats location. It also acts as an EPIRB in emergency situations, and alerts of high water and low battery etc. Sort of an On-Star for boats.
The Stolen Boat Monitor (geofence) works, I have purposely left the dock without turning it off to see what happened and within a few minutes got a call from SeaKey on my cell.
I get a discount on insurance by having it which pays for the monthly service.