Physical security is a great deterrent for opportunists. For targetted attacks (like the recent thefts) it's a time delay. If you've got a boat that's attack proof for 5mins, you've done a bloody good job of your security imho. I recommend a visit to Bosnianbill on YouTube if you're interested in lock security or starting lock picking as a hobby. Look up 'Bump Keys' if you want to see how easy it it to get into bad locks.
Noisy alarms with simple electronics, you don't need anything too fancy, it's more important that they're a pain in the arse (E.g 'Alarm + Timer = AGHHHH'). I was going to ask if there were any other electronics enthusiasts on the site? This would be a fun Arduino/PIC project to share with the forum.