I'm sure you can pick up some cheap GSM module on eBay and wire it to the siren output
The main issue is when the boat is on the mooring, and when its not being used for extended periods of time, the boat is left on a trailer on private land, well away from the eye of the public, so my main issue is when its on the mooring, because thats when the most of the theft is happening.
I might pick one of these up (they are only £24) and see how much it draws, to be honest, I can always improve physical security rather than going to this