In places where I worry about the dink, I use a thick bike cable to a lock that goes through the motor clamps. This secures the motor to the dink, and the motor and the dink to that cable that I run, to what ever strong attachment I can find.
Metal cleats on a dock or the mother ship are the best thing to lock to... I pass the cable through the center of the cleat and then the cable back through itself and lock it to the motor clamps. You can get those thick rubber coated metal bike cables in a wide variety of lengths.
I started doing this in San Diego when I heard of dink thefts at marina docks of what should have been fairly secure marinas. Turns out the thieves had a small boat and would just enter the marinas from the water and find dinks just tied up... wooosh they were gone with a quick slice of a knife. Yeah, those guys were caught... but that isn't to say other jerks out there don't exist.
Now a bike cable is still not theft proof (as any cyclist will tell you) a cable cutter can make somewhat quick work of them... But the hassle, more than likely, causes the potential thief to look elsewhere.