Toronto filmmaker and conservationist Rob Stewart is missing after a Tuesday night dive off the coast of Florida.

The U.S. Coast Guard is on the scene Wednesday morning to search for Stewart, 37, who vanished while diving near Islamorada in the Florida Keys, a chain of islands between the Atlantic Ocean and Gulf of Mexico, located nearly 200 kilometres off the state's southern tip.

"We're in the critical window of daylight right after he's gone missing where, if we're going to find him on the surface — where we'd like to find him — it's going to be today," Stewart's sister, Alexandra Stewart, said in an emotional interview with CBC Toronto on Wednesday morning.

The family has also launched a GoFundMe campaign to mobilize more resources to help with the search, she said.

Stewart is best known for his 2006 documentary Sharkwater, an examination of global shark-hunting and its impact on the ocean ecosystem, and was active in underwater filming.

His sister said his latest dive was part of work on the film's upcoming sequel, Sharkwater: Extinction.

Jeremy Weaver, senior chief of the U.S. Coast Guard, told CBC Toronto that a helicopter from Miami, a boat and a team of divers are involved in this morning's search.

Weaver said Stewart was "diving on a wreck off of Islamorada" with three other divers when he went missing, but the three are safe.

GFX Map: Islamorado, Florida

Not clear why Stewart disappeared

Stewart "resurfaced at the end of the dive, and as the boat was turning around to pick him up, he went back under — and was not seen again," Weaver said.

It's not immediately clear what caused Stewart to go back under water, but his sister said it was a "particularly difficult" dive, going to a depth of nearly 70 metres.

She said her brother may have lost consciousness after doing a third dive that day.

"It's extremely rare that even experienced divers are qualified to do that kind of dive," Stewart's sister said.

"The other fellow who was on the same final dive appears to have lost consciousness when he surfaced, so it might have been too much diving in a certain window. It's hard to speculate."

Weaver said weather conditions were good at the time of the dive early last night.

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Stewart is shown during the Toronto International Film Festival in 2013, when his film Revolution was featured. (Canadian Press)

Michael Parfit, an environmental writer and filmmaker based in Vancouver, said Stewart routinely takes risks while diving because he has been "so driven to know these animals and transmit what he knows to the public."

A deep dive comes with potential dangers, said Parfit, but it's "nothing unusual."

Family in Florida as volunteer help sought

Stewart's sister, who is currently in Toronto, said the siblings' parents and her husband are in Florida right now and in constant contact with search teams.

"If there are other people out there, what we desperately need are more surface searchers," she said.

Alexandra Stewart1:11

Stewart's sister stressed his love for the environment, something evident in his work as a filmmaker.

"He has a deep love for the ocean, and wildlife, and our planet and people, and trying to make the world a better place," she said, her voice breaking.

"And he really saw this as where he could make his unique mark, and I think we've seen that over his life. What he's done for shark finning and illegal fishing has been tremendous."

Dustin Titus, who has known Stewart for more than a decade, also praised his passion for conservation.

Stewart's work resonates "around the globe," said Titus, a member of the board of directors for the United Conservationists Society of Canada.

The conservation community is experiencing "a lot of nervousness" over Stewart's disappearance, he added. "Everyone's really scared."

Stewart's films have won dozens of awards

Sharkwater premiered at the Toronto International Film Festival and has since won more than 40 awards at film festivals around the world. 

Stewart's second film, Revolution, was the highest-grossing Canadian documentary in 2013 and won 19 awards from global film festivals.

Stewart was born and raised in Toronto, and studied biology at Western University in London, Ont.

He is also considered one of the "distinguished alumni" of Toronto's Crescent School, which he attended from grades 7 to 9, and where he has since returned to speak to students about his work in marine conservation.

"Our thoughts are with Rob's family and we share their hope for his safe return," the school said in a statement.

"We are looking forward to having him back because the world needs him," echoed Parfit. 

"The sea, the oceans need him."

Sharkwater film

Sharkwater was a runaway hit, winning dozens of awards and raking in more than $1.6 million US worldwide. (Freestyle Releasing)

With files from CBC's Linda Ward