Scotty, the SPOT is designed as a one-shot device, essentially the batteries will last maybe one week and you've got to manually press the button at least once a day. In order to do what you want to do, you'll have to go out to the boat every day, rain and shine especially rain, and change the batteries at least once a week. And IIRC it prefers Lithium AA's, which may be something like $10 for a four-pack. Or, you can hack into it, totally voiding the warrranty, to apply an external power source and a daily trigger to push the "track" button. Easy enough to do--if you're into electronics.
OTOH devices likethe zoombak already are built for vehicle use and constant use and the geofencing is including in their firmware load. (Geofencing being the standard term for "anchor watch" and similar.)
100 yards is tighter than you think. You've got a normal swing on your anchor or mooring, that might exceed fifty feet. Then there's a normal fluctuation in GPS, even with WAAS GPS, which I don't know if either unit uses. If you want to pin the locaiton down tighter than 100 yards, expect false alarms. Or, a SEAL team laser-targeting a Predator drone on it. :-)