The sheriff's office said its bomb squad was summoned to examine the cylinder, which "appeared to have been submerged for a significant period of time."
The object was identified as a M122 World War II-era flare bomb, designed to create intense light for photographic missions.
MacDill Air Force Base's explosive ordnance disposal team was summoned to the scene, and members of the public were kept 1,400 feet away from the object on each side. Officials said about 25 homes were evacuated.
St. Pete Beach Mayor Maria Lowe, who was present for the demolition, said measures were taken to ensure no sea turtle nests were disturbed. She said the nonprofit group Sea Turtle Trackers was on hand to monitor the nests.
The bomb was detonated with C4 just after 5 p.m.
"I hope you've had a great day at the beach with a little excitement," Lowe told the assembled crowd before the explosion. "I don't know if I'm going to have the opportunity to do a countdown, so please do not be taken off-guard if you hear a large boom."
