In regard to truly high end watches my experience has been with : Rolex, Patek Phillippe, Baume Mercier, Cartier and others, the main savings will be in tax. You will have to buy them from a duty free store but remember you STILL have to declare it or them. If you have to declare it and it wasn't purchased then it depends on the price of the watch and the duty that would be paid. This may negate any tax savings.
The actual pricing is not going to be that great because most of the pricing is controlled by the manufacturer and if you see a too good to be true price you better think twice, especially if it is not a large store such a DI or Kirk's.
One advantage is that you may find models that are not common in the U.S. or you may find discontinued models.
Don't try to wear it through customs unless it's declared and you have the paperwork. A new, high end watch, can be easily spotted. Been there done, that and I now wear the old one when getting off the ship.