Paul,
Sorry to hear about the theft - I cant imagine how you must feel!
Whilst at home I regularly help out in the Poole Harbour Watch program, and really feel that this is the only way we can stop the thieving scum...
Could you perhaps chain from your mooring buoy? (though bolt cutterable..) or a way to immobilise the engine when you are not there?
The problem with an alarm, is the size of poole harbour, and the lack of any next door neighbour to report anything , though it may startle someone...
I hope it gets sorted okay with the insurers!
Dan