There are some in Key West but they are not through Disney. You would have to book on your own. Island Water Sports is one place and Key West Water Tours is another. I have not done this so I don't have a review for youo but I am looking into it for next year.
We rented jet skis on Grand Caymen, took a taxi to the 7 mile beach, it was really cheap, maybe $6, then rented skis on the beach, because we were with Disney (bring your KTTW card) we rented for $45/half hour, we had a blast.
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