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A Sydney businessman celebrating his wife’s birthday has fallen to his death from Royal Caribbean’s Symphony of the Seas in the Caribbean.

Jonathan Pfahl, a former Sydney’s Knox Grammar School student and founder the British-based company Rockstar Group, an investment and mentoring organisation.

Mystery surrounds how Mr Pfahl could have fallen from the megaliner. An investigation is underway.

It is believed he drowned after falling overboard as the ship made its way from St Kitts to St Thomas in the Caribbean early on Wednesday morning. Rescue crews from the ship recovered his body.

“Upon the initial report that an Australian adult male had gone overboard, our ship immediately turned, launched a rescue boat, and conducted a full-scale search,” Royal Caribbean said in a statement to media.

“We are deeply saddened by the tragic incident that occurred on the Symphony of the Seas en route to St Thomas. Our hearts go out to the family and loved ones for their loss, and our care team is providing support and assistance during this difficult time.”

The 6680-passenger ship is on a seven-day cruise from Miami through the eastern Caribbean.

A friend close to London-based Mr Pfahl, who grew up in Sydney, told news.com.au he was holidaying with his wife and two young children at the time.

“It was his wife’s birthday family holiday,” the friend said.

“All we know is he went overboard early (in the) morning and the body has not been released off the ship in Miami yet. He was a businessman who helped so many start ups around the world,” friend and co-founder of AmeriCamp Lee McAteer told news.com.au

“He helped businesses get off the ground by helping to provide loans to help get the business started. I was introduced to him in Liverpool (in the UK) and we hit it off straight away.”

“The world has lost a brilliant, innovative and caring family man who was an inspiration to everyone around him.”

The Department of Foreign Affairs and Trade said it would provide assistance if called upon. Because the ship was registered in the Bahamas, US authorities said they would not investigate.

Over 20 people went overboard last year.