Ive just bought myself one of THESE
It is a GPS reciever with a GSM modem in one.
It takes a pay as you go SIM card so if your pride and joy gets nicked you just send the boat a text message and it replies with is exact posistion!
You can also phone your boat and listen in to the crooks chit chat via a hidden microphone!
If the boat is placed indoors where there is no GPS reception you can always track the GSM signal using one of the many website such as www.followus.co.uk
It also has various input/outputs so you could set up a alarm trigger which would send you a sms when triggered, or maybe imobilise your engine.
The whole lot including GPS/GSM ariels cost me £140 not bad. My insurance company also gave me a 10% discount.
All ive goto to do now is find a nice waterproof box and somewhere to hide it in the boat.