Mike's advice on buying a high end watch and dealing with customs is very good. I would add a couple of points.
First, know what you are buying, especially if you are looking for a Rolex watch. There are some very good Rolex knockoffs out there that can be purchased for about 10% of the cost of a genuine one. They are tough to spot and I know one person who got burned in Nassau.
Second, know what you can buy the same watch for in the US and consider the warranty. There are advantages to buying a high end watch at home. Unless the savings are significant, you should consider buying from a local dealer for service and valid warranty. A lot of Swiss watches will not come with a US warranty if purchased outside of the US. You will not be able to get warranty service in the US without a US warranty.
Third, and some may dispute this one. I would never make a major purchase like this with cash. Always use a credit card. If there is a problem with the item, its good to have the credit card company on your side. American Express is particulary good at merchant compliance. Plus, if your outside the US, you will get the correct currency exchange rate, not whatever the merchant wants to give you.
Fourth, if you already have a high end watch, consider leaving it home when traveling outside the country. I have been hassled more than once by customs while wearing a nice watch back into the country. Even though I already owned it. You can declare it with customs before leaving, but thats more hassle than just leaving it at home and wearing a Seiko.
Personally, I have found St. Thomas, USVI to be an excellent place to purchase high end watches. The savings can be significant and you get a US warranty. I have actually saved enough on a watch purchase to fully fund the long weekend in paridise for my wife and I.