Get one of THESE
Runs of 12v,uses a combination of GPS and GSM. takes a pay as you go sim card, you can set up a "Geo Fence" around your boat of say 200 metres. if it notices the boat move outside that area it sends your phone a text message with its exact latitude/longitude which can be tracked on a website for free such as multimap.
The accurcy is scary. played hide-n-seek with the missus when i had it in my car as a test, she pinpointed me exactly in the pub beer garden!
Ive now made a battery backup system which self charges from the boat battery in case they were to dissconect the boat battery, it will keep the unit running for 24 hours. It also texts me if it sees the boat battery dissconected too.
Mounted it in the rib now hidden away, the gps seems very sensitive as it still gets a good signal inside my garage.