When I've spoken to people about securing boats, the general consesus has been that it is impossible to ever prevent your boat being stolen. All you can do is:
a) Slow any thief down - the idea being that the longer they have to spend getting past your security the more likely they are to either be noticed, or get jittery and leave.
b) Make stealing your boat harder/less attractive than stealing the one next to it! Scuff up outboard covers, have some nice pretty high visbility security, and hope they don't even bother.
There is no security method that is absolutely foolproof and can't be broken, so go for the best, and the most visible you can afford. There's also a fine line between not enough security, and so much security it takes you 2 hours to get your boat on the water!
The most humourous hidden security device I've seen is where a boat had 2 highly visible chains locking it to a fence, and a wheelclamp. They were clearly visible. What wasn't, was the chain connected to the trailer underneath the boat, onto an eyebolt set very firmly into some concrete. The chain was pretty invisible due to grass and such, and was about 3metres long. The idea was that someone got past the other security, hitched the trailer to their car, had 3 metres to build up speed, then totally buggered their towbar/the trailer. The chain used was about 2cm thick toughened chain, shackled to the eyebolt, and shackled round as much of the trailer as was possible.