Best idea I've seen is to fit an oversize drain
plug below the waterline. Secure the dink on shore, or on a dock, and take the
plug with you. Now it simply can't be launched without sinking it.
GPS in the
engine? Would
work if the engine cover was plastic, and you supplied
power to the tracker. $150-200 would buy you a Zoombak, which is about the cheapest, plus another $150 for the annual
contract on it.
You can do something direct with APRS, if you are a licensed ham
radio operator, but that will still cost you a couple of hundred to set up.