I rented a house in Islamorada in the Keys 4 years ago. I used VRBO, which is a website that markets private owned properties. It was an awesome house, with a 150 foot private beach, kayaks, snorkeling gear, swimming pool, and bikes. The house itself was around 5000 square feet, with room to sleep 8. It was $4800 for the week. Beautiful place.
The website has nice search features and good filters so you can get pretty specific, and most homes provide good photos of the property. You can also see available dates. Most place require a minimum stay of 6 to 7 days. Once you've decided on a place, VRBO will put you in touch with the home owner and you deal with them.