Contact Dave at Key West Kayak Fishing. Spending a half day with him will educate you enough to venture on your own. He offers eco tours/fishing and can really put you in some nice paddeling.
Bahia Honda S.P. at mile marker 35 has sit on top kayaks for rent, real nice paddle around the island for a day trip. Also has the best beach in the keys.
How long are you there? Fishing?
                               Jeff