Joseph Dragojevich scuba dove into a 510-foot sunken Navy ship, the USS Spiegel Grove, on Thursday and hasn't been seen since, according to the Coast Guard.
The ship became an artificial reef after it had been sunk in 2002.
A search party has been looking for Dragojevich since his friend reported him missing.
Dragojevich, 43, who is a captain with the Lake County Emergency Medical Services, was diving off the Florida Key with his friend James Dorminy, 51.
The two had chartered a boat with the Key Largo-based Scuba-Do Dive Co., with six others.
According to the Miami Herald, the two friends went diving into the wreckage without the assistance of the dive company or with anyone else from the charter.
Dorminy told authorities that the two used reel lines to dive into the complex parts of the ship.
At one point, Dragojevich signaled to Dorminy that everything was OK before they dove farther. But the next moment, when Dorminy looked back at his friend, Dragojevich was gone, his reel line gone slack.
Dorminy swam back to look for him but found only tangled reel line and no sign of Dragojevich.
The ship is a vast labyrinth of narrow passageways and blocked-off areas. Dorminy told authorities he searched as many areas of the ship as he could, frantically looking for Dragojevich.
For now, the Monroe County Sheriff's Office Dive Team and the Water Emergency Team of the Key Largo Volunteer Fire Department Dive Team are conducting a search of the ship. They're searching the outside, as well as the inner, more-complicated parts.
This isn't the first time someone has vanished diving into the USS Spiegel Grove. In 2007, two divers from New Jersey died inside one of the vast ship's rooms. A companion of theirs died soon after when he attempted to reach the surface.