Stewart, 37, had been drysuit-diving using a closed-circuit rebreather, filming a steamer wreck called the Queen of Nassau that lies at a depth of 70m.

He was diving from a boat called Pisces with three other divers, who are all said to be safe. The wreck-site is near Alligator Reef off Lower Matecumbe Key, and weather conditions were said to have been good.

Boats and helicopter searches are continuing, with divers from Key Largo Volunteer Fire Department arriving at the scene this morning (1 February). The US Navy, Florida Fish & Wildlife Conservation Commission, Monroe County Sheriff’s Office and Sea Shepherd have also joined the US Coastguard in the search operations.

An experienced diver, Stewart has been an instructor since the age of 18.

A Coast Guard representative told CBC Toronto that he had resurfaced at the end of the dive but, for unknown reasons, had gone back beneath the surface while the boat was manoeuvring to pick him up, and had not been seen again. Later reports have suggested that the first diver to come aboard had passed out.

Rob Stewart trained as a biologist and became an influential environmentalist film-maker.

Sharkwater, his film about the human threat to sharks and the adverse effect of shark-finning on ocean eco-systems, took four years to make, involving extensive travelling and undercover filming.

It was released to acclaim in 2006, winning more than 40 global film-festival awards and proving highly influential in the worldwide campaign against shark-finning. The follow-up Revolution built on Sharkwater's conservation message.

Divernet - The Biggest Online Resource for Scuba Divers