A GoFundMe page says he was "scuba diving on the queen of nassau wreck, six miles off shore of Islamorada Florida, when he disappeared after surfacing" on Tuesday.
The news came as Coast Guard officials were set to suspend the search efforts at sunset on Friday night.
Tyler MacLeod, a close friend of Stewart's, said the costs of sending out private aircraft and boats have reached over $100,000 so far, and they'll need more help if the coast guard calls off its search.
A Facebook post by Stewart's sister, Alexandra Stewart, asked for privacy in order to grieve.
Stewart rose to public acclaim upon the release of his 2006 film Sharkwater which helped raise awareness of shark finning which lead to the eventual bad of the practice in 2013. But after emerging for the third time, Sotis began to lose consciousness. He had been working on a sequel to his popular documentary Sharkwater at the time of his disappearance.
Stewart said he and his colleagues risked their lives to make the film: they visited a Costa Rican warehouse that trafficked in illegal shark fin and confronted poachers on the high seas.
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Rob Stewart will be missed.
The filmmaker was accompanied by a group of divers to shoot the follow-up film to "Sharkwater" when he went missing after briefly resurfacing and possibly drowning away.
Rob Stewart went on to study marine biology and zoology in college and became passionate about conservation.
"If anybody can survive in the water, it is him", Brian Stewart, his father said in a telephone interview, adding his son was "super fit" and a highly skilled scuba diver.
He was also known for his documentaries including "Revolution" and his memoir "Save the Humans". "So it's going to be a really dramatic century unless we do something about it".
In 2011 municipal governments across Ontario, including Toronto and Mississauga also voted in favour of banning the sale and possession of shark fin.