The explosion occurred just before 7am yesterday as the diver walked to the front door of the ground-floor apartment in St Petersburg, intending to load the cylinder into his car ready to go diving with friends.
The force of the blast was enough to blow out the door and living room windows, and the windows of nearby cars. Assorted debris was hurled some 30m from the building.
Another man and a woman were in the apartment. They were in a different room when the explosion occurred and were not hurt.
Emergency services took the gravely injured diver, named by regional media as 23-year-old Russell Vanhorn II, to St Petersburg General Hospital but he could not be saved.
Cylinders remaining in the apartment were checked as safe by Tampa Bay Regional Bomb Squad.
Remains of the blown cylinder, reported to be of aluminium construction and to have split vertically down the middle, are being examined.
St Petersburg police are investigating the history of the cylinder, including its servicing and last fill.
Cylinder explosions can occur due to corrosion from inadequate maintenance, a manufacturing fault or overfilling.
A US Marine from 2005 to 2009 and an Iraq veteran, Vanhorn reportedly learned to dive during his last battalion placement in Japan. The sport became a passion after he left the military.
He moved from Iowa to Florida where he qualified to work as a sport diving instructor and reportedly developed an interest in cave diving.
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