A devastating boat explosion at a popular tourist destination has left 11 people hospitalised, with one witness describing how the blast hurled several individuals into the sea before flames consumed the charter vessel.

US authorities have declared a level two mass casualty incident after emergency services deployed more than two dozen rescue units to waters off Miami's coast to assist those wounded in the explosion.

The charter vessel was operating in Biscayne Bay, close to the Sunshine State's renowned Haulover Sandbar, when the detonation took place.

According to the Miami Herald, which obtained audio recordings of dispatch communications between headquarters and emergency crews at the scene, there was considerable confusion when rescuers arrived.

The dispatcher attempted to establish the number of casualties, with the firefighter responding: "They keep coming.... we just aren't sure right now", reports the Mirror.

The audio confirmed that numerous individuals sustained burns. Several were propelled into the water by the explosion and were retrieved by fire-crew lifeboats.

The casualties were subsequently rushed to nearby fuel docks for transport to local emergency trauma facilities, with some airlifted to hospital. Several were taken to Jackson Memorial Hospital's Ryder Trauma Centre and Burn Centre, the publication reported.

The Miami Herald identified the vessel as a Press Cruiser 400 Express, a 40-foot cabin cruiser named Nauti Nabors, registered in Texas. The boat's capitain was unharmed by both the explosion and subsequent fire. South Florida boat captain Patrick Lee witnessed the horrifying incident unfold firsthand. He was ferrying passengers aboard his tiki boat to a larger vessel moored in waters near the Haulover Sandbar when the explosion erupted nearby.

"We saw three people fly out of the boat," he said.

Lee recalled that the boat was engulfed in flames within minutes, noting that several victims were carried away on stretchers by paramedics.

"I just saw a lot of them with burns all over them, and I said that it's a hot fire," Lee said. "It is quick, but it's high octane gasoline, and so a lot of them got burnt, and it was an explosion."

The seasoned boat captain, with more than 50 years of experience, suggested to local media that the blast was most likely triggered by exhaust fans that had not been switched on. That would have allowed fuel fumes to become trapped aboard and, when the key was turned over in the ignition - then boom.

"[The captain's] people got on board, he turned the key, didn't open the hatches, and didn't turn on the blowers - he blew people out of the boat," he said.

He added: "That's the worst thing, [it was totally] preventable. I find most tragedies are from a lack of education. Many people out here shouldn't be operating boats."

He added that he based theory off what he had personally observed and his five decades on the water. A passenger who was on board during the blast confirmed a similar story to the Miami Herald. Leo, who did not provide a surname, told the paper the explosion was caused by a gas leak, adding: "It was the worst experience of my life."According to dispatch audio logs, fire crews indicated the explosion possibly originated from the boat's fuel tank.Fire and rescue officials have subsequently issued a public statement, with the boat explosion serving as a stark reminder to "prioritise [their] safety while out on the water".In a statement, the Miami fire service said: "Make sure your vessel is equipped with a working fire extinguisher and all required safety equipment. Before departure, perform a thorough safety check to ensure your boat is in good working condition and free of potential hazards.

"Taking a few extra precautions can help prevent emergencies and keep everyone safe." The explosion and resulting boat fire is now under official investigation by the authorities, who have yet to establish the cause.