This reef is right on the edge of the barrier reef, and to reach it you catch a boat tour from the Marathon area on Vaca Key.
This is a popular spot, with lots of mooring pins and boats. It is marked by a good sized lighthouse.
Sombrero Reef Marathon Wiki
Sombrero Key Light is located near Key Vaca in Marathon, Florida. The lighthouse is located on a mostly submerged reef.
The name Sombrero Key goes back to the Spanish, and old charts show a small island at the spot, but by the later 19th Century the island had eroded away, with some parts of the reef exposed at low tide.
As a result, the reef and the lighthouse have also been called Dry Banks.
The lighthouse was put in service in 1858, automated in 1960, and is still in operation.
The foundation is iron pilings with disks, and the tower is a skeletal octagonal pyramid of cast iron. The Sombrero Key Light is the tallest lighthouse in the Florida Keys. (via Wikipedia)