I found this one place behind the Holiday Inn on Islamorada Key where they let you take their jet ski's anywhere you want to go for $55/hour. (Most rental companies make you stay within their sight.)

It was so much fun riding out in the ocean with no restrictions on where I could go.

I spent the afternoon riding around the Dolphin Research Center at Grassey Key, and I also rode out to the "crown jewel" of the Keys called The Reef. (But I didn't have any snorkeling gear, so I just watched as other divers went hunting for buried treasure.)