Alexandra Stewart said her brother Rob Stewart, 37, was filming "Sharkwater: Extinction", the follow-up to his documentary - about global shark-hunting's impact on the ocean ecosystem - near Alligator Reef on Tuesday around 5:15 p.m. local time, when he disappeared near Islamorada, located almost 200 kilometres off Florida's southern tip.
"It was the third dive of the day, I believe it was just him and another very experienced divemaster but it was a very hard and technical dive", Alexandra said.
Stewart directed the 2006 documentary "Sharkwater".
"HELP NEAR ISLAMORADA. FLORIDA - PLEASE HELP WITH A SEARCH AND RESCUE FOR ROB STEWART WHO IS LOST DIVING", she wrote. The diver collapsed unconscious and began receiving oxygen.
The search for Stewart includes boat and helicopter crews and hasn't stopped since Tuesday afternoon.
The family is in shock, she said.
"But that said, we were surprised that both he and this other diver suffered any sort of ill-consequences from this dive and it's hard to speculate about what might have happened there, but diving also has some inherent risk in it, unfortunately".
Jeremy Weaver, senior chief of the U.S. Coast Guard, told CBC Toronto it's not clear what happened to Stewart following the dive.
"As long as there's a chance of survival, we're going to search", Cangemi said.
"Time is really important right now", she said. "We remain hopeful that we'll be able to find him alive".
The U.S. Coast Guard launched a search for him today (January 31) near the Upper Florida Keys.
Stewart is a wildlife photographer, filmmaker and conservationist.
Mr Stewart was diving about six miles off the coast of Islamorada, in the Florida Keys, according to his friends. Details on his FaceBook page.