Similar design to the Nancy Drew watch, and dated 1977 and Universal City Studios, Inc. The band is different, but it's for a boy, so that's not a huge clue or anything.
Now to just find out how it was sold! My original thought was that it might have been sold at Universal Studios--maybe in the gift shop--but I checked out some of my other TV show collectibles and noticed the puzzle and the lunchbox all have the same designation of Universal City Studios, Inc. Sears did have a clothing line featuring Sean Cassidy on most items around 1978, so perhaps the watch might have been offered in one of the catalogs--Sear's, Pennys, etc..
I'll keep digging...