I have one of these:
TK103 B Car Vehicle Real time Spy GPS Tracker Remote Control | eBay
I added a shock sensor:
NEW!!Shock sensor shake sensor for GPS tracker GPS103A,B,TK103A,TK103B,best qual | eBay
And mounted it i a water tight plastic box with a gland for the power cable.
The device is set to sleep until it detects vibration then it will text my phone with the alarm description and its google map link location.
Then i can track it live for this website (for free)...
GPS-Trace Orange 2.0
It works amazingly well and has so far been 100% reliable.
The unit runs from the boat's battery, but if disconnected it will run from its own internal battery for a day or two.
I use a giffgaff SIM card. Its costs nothing to run unless it texts you an alarm (6 pence).
If you wish to have live GPRS tracking on permantly it costs 20p a day in data, but this will also increase battery drain so not ideal if you dont use the boat for a couple of weeks.
My boss is so impressed we have been trialing one in our delivery van for the last 2 months, and now i've bought 10 more to fit in our fleet vehicles.