I run a company called Touchline Security. We are a small UK based company designing GSM security systems and have recently completed the design of the Nautilarm Marine Security System.
The system offers wirefree sensors, 12V operation, low power consumption, low initial cost, no running costs and GSM paging built in. It has been designed with boats in mind and can be supplied with a discrete panel mounted antenna to go on the roof for good reception. There is also the option to add GPS for remote tracking.
http://www.touchline-security.co.uk/nautilarm.shtml
Please contact me for more information or if you have any queries.