Key West, Florida
If you are a weak swimmer or have never snorkeled before, only go if the conditions are excellent. Only 25 of the passengers went snorkeling due to the literally 10 to 15 foot waves around the boat at the snorkel site. All of them returned to the boat within 10 minutes due to the high waves and lack of visibility.
Also the kayaking was pretty lame. You only paddle along the coast of the mangrove, not very far from the boat, less than 5 minute continuous paddle. In fact, the kayak guide just sends you back on your own to the boat and waits for the next group to paddle out to her - that's how close it is.