There is a Hobiecat dealer in Key West:
Hobie Cat Dealer
6300 3rd St Key West, FL 33040
You can try calling them, I have a house down there ( in old town) and have talked to them before. I think they are associated with Sunset water sports (a really great bunch of people).
I know for sure they have Kayaks and Hobie cats for rent down at Smathers beach (on the south side), I believe they also have more at the Parrot Key resort (on Roosevelt just when you get into town).
We go down there on average about once a month and I always take my Hobies along. We have had an Oasis, Revo's, and now TI's. Most of the time these days we end up launching down at either Smathers or Higgs beach and go just offshore (it's great snorkeling and diving anywhere on the south side, near FT Zachary Taylor park, but be careful of the strong current at the point. Also around Tank Island or out anywhere in the Mule keys (but you would need a pretty big boat (like a TI) all properly ocean rigged to go out to the Mule keys, our favorite is Cottrell key which is about a 15 mile haul from the Simonton street launch. We also like the areas just up north around Horseshoe key, and we have spent a lot of time up and around Islamorada and Key Largo (John Pennekamp is the scuba diving mecca of the world if your into that stuff).
Be careful down there the currents are very treacherous and you can easily be pulled out to sea (start learning Cuban now just in case LOL), be sure to ask where it's safe to go. A good example is the area in front of Mallory square (on the west side), the currents there run 6mph if you launched there you would never get back unless you know the tides and currents (also the cruise ships will chase you off of you go anywhere near them, they are very dangerous to be close to).
I'm sure they have Hobie Waves, Getaways, etc for rent at Smathers, I'm pretty sure they also have Oasis and a couple others available, but unfortunately no AI/TI boats for rent that I have seen anyway.
I think Vabeach1 was talking about Robbies (just south of Islamadora), that is one of the coolest places in the Keys and they do rent Hobie Kayaks, if you go to the restaurant make sure to have the Lobster bisque. But that's like 80 miles from Key West (kind of far).
We will be in Sarasota that weekend for the superboat races (I'm and ex-powerboat racer) (http://superboat.com/2013-sarasota-schedule/