My husband did the SUB Bahamas excursion. He liked it, but said he wouldn't do it again. There is a shark encounter that he wished he had done.
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The snorkeling is great at Cococay! We brought our own gear and only had to rent the mandatory life vest.
We had a great time just walking around Key West. There is a sandal factory that makes leather sandals right on the spot. My husband and I bought three pairs. The Conch train is nice if you've never been. There is a lot of history in Key West.