Get about 20 rolls of duct tape...after that much tape, theives are either not gonna recognize it or get caught in it like fly paper when they try and cut it open. Problem solved.
I used to rent jet skis and have moved them around with just one other guy. Of course, that wasn't a great distance (trailer to trailer), but those of you who've met me know I'm not what they call.....big. So if one other guy and I can move one around, a couple real determined theives could carry one off without problem. Chain that sucker to the trailer, and chain the trailer to something big. And then put it all in your brother's backyard so no one sees it.