The USN did similar explosive tests off the West Coast in the mid 60s. Their purpose was to see how new ship configurations could withstand a nearby nuke blast. I was on a WW2 heavy cruiser that they converted to missiles. They removed the guns and steel superstructure above the main deck, & installed missiles and an aluminum superstructure. Our ship took a 48 degree roll from the blast.
They also took a WW2 light cruiser and replaced the superstructure above the main deck with 2 different design DLG superstructures. The crew went below the main deck for a very close test blast. After that test, the ship was scrapped. I heard there was a 12" diameter divot in the 4" thick main deck from a rock that came down.
These tests were conducted in shallow water of San Nicolas Island, which is off the coast and is part of the Navy's Pacific Missile Range.
I expect the Florida blast location was as far as possible from the mainland, while at a water depth necessary for the tests.

NOT a Conspiracy.