I don't know where to start.
1, Do you seriously think US customs is concerned about someone buying a watch for a friend and calling it "re-sale"? Thanks for the laugh. We've been on 40+ Caribbean cruises, taking multiple swiss watches along (Omega, Breitling, etc) each trip and buying new ones also, NOT ONCE has customs ever looked at a single watch.
2. "Many" watches in the Caribbean being knock offs: Maybe in straw markets or whatever, not in reputable jewelry stores. Most stores are authorized dealers for what they sell.
OP - do some research on jewelry stores where you're going in the Bahama's. But generally speaking St. Thomas, St. Martin and others are the best for both selection and price.