Was just in Key West last week Friday and it was 75 degrees, it was 51 just two days prior to that. Not sure when you're going, but the water might be a little chilly
We rented bikes and rode around the entire island. We stopped off at Hemmingway's and also rode past a beautiful beach on the south side of the island. We did all this and still had time for lunch and shopping.
Have a great time!