The Spiegal Grove was a navy LSD (landing ship dock) that was to be sunk off Key Largo to become the largest vessel ever used as an artificial reef.
There were a few technical difficulties though, and the 510 ft. ship sunk a bit early, in the wrong position. Now the bow sticks above the waves, the Coast Guard is patrolling the area to keep the incautious safe, and it's going to be a bit before she's righted.
Ocean Divers of Key Largo
has a website about the Spiegal Grove, which explains it all in detail.