I would look for a device that uses the Globalstar
network. The SPOT would work but it requires the person on the boat to interact with the transceiver for it to work. My guess is you are looking for something that transmits the boats position automatically on a frequent and consistent basis.
I found a couple Globalstar
based services by doing a Google
search. I have not used either so I cannot vouch for them.
AeroAstro - Asset Tracking and Remote Monitoring Solutions
Worldwide GPS Satellite Tracker SX1
Globalstar - Wikipedia, the free encyclopedia
I'm curious, as well as others I would imagine. Would it be okay to ask for a few details about how you plan on using the tracker?