I have a tracker on one of my cars, the insurance company insisted on it and there was no discount (either fit it or don't take our insurance). It does have a small back-up battery, however it needs constant power and is always on a trickle charge in the garage.
It will email, text and phone if the car is tampered with, moved, driven over a certain speed limit etc. I can also disable the car through the web interface (turn the engine off, sound the horn, flash the lights). There is a also a live mapping facility showing where the car is 24/7.
I doubt any of this would be available from a device that would fit on an outboard in addition there is the problem of putting it somewhere that wouldn't be spotted.
The manufacturers ought to be trying to find a solution to this - it would not be beyond their wit to fit a tracker deep in the engine somewhere (encased in the leg somewhere perhaps) and being recharged when the engine is run and supplied with voltage through the engines electrical circuit. I guess a power pack could be left on the boat to supply power.....
I can imagine how deeply irritating it would be to have an engine stolen and the boat vandalised to remove it.