Boating in the Keys can be tricky due to reefs and turtle grass. Not only can these damage your boat, but the waters off the coast of the Keys are a protected Marine Sanctuary and you can be fined for damaging reefs and turtle grass. So use caution, get a good chart, learn what the channel markers mean, and plot your route carefully. Sometimes the best way to your destination is not a straight line!
Robbie's rents boats, fishing equipment and snorkle gear. You can buy bait and beverages, too. Boyphoenix and I did all of the above and off we went. Our boat was a 17 footer, I think, and very utilitarian. Not much space to stow our stuff or for sitting, but it got us around.