As the sun set on Wednesday, the family of Rob Stewart remained hopeful he’d be found, after the Toronto filmmaker and environmental activist disappeared during a dive off the coast of Florida on Tuesday.

A spokesperson for the U.S. Coast Guard said efforts to rescue Stewart would continue throughout the night and into Thursday morning.

“They’re trying their utmost to be as positive as possible,” Victoria Gormley, a spokesperson for the family, said Wednesday evening. “They’re clearly devastated. This is a scary situation.”

Stewart disappeared Tuesday following an incident that left his diving partner unconscious, his sister said.

Stewart, 37, was in Florida to film a documentary called Sharkwater: Extinction when he apparently slipped under the water during a dive near the Florida Keys.

“He had surfaced and gave the OK sign and then he was gone,” his sister, Alexandra Stewart, said Wednesday.

“The other diver got on board and was struggling to get on board and then fell unconscious,” she said.

The crew of the boat lost sight of Stewart amid the commotion, she said.

Stewart’s sister said there’s a concern that her brother lost consciousness as well.

“We’ve heard that people can survive more than 72 hours in these waters,” she said. “It’s still a surface search, which is great news.”

The dive was her brother’s third of the day, she said, noting that he and his diving partner had gone down about 70 metres.

Stewart is an experienced diver who started underwater photography at age 13 and had qualified as a scuba instructor trainer by 18.

“He’s got everything going for him in terms of his ability in open water,” said Gormley.

His family is in shock, Alexandra Stewart said.

“I’m doing terribly. We all are, but we have hope.”

She said her parents and her husband are in Florida helping to manage search efforts, but they still need those with experience to join the search that is being co-ordinated by the U.S. Coast Guard.

A gofundme.com site has been set up in case a prolonged search needs to be mounted.

“The coast guard has been doing an excellent job of leading the search,” Alexandra said.

The search team is using a helicopter, boats and divers, and includes members of the U.S. navy, the Florida Fish and Wildlife Conservation Commission and the Monroe County Sheriff’s Office.

“They’re going to be out there all night long,” coast guard spokesperson Jonathan Lally said.

Earlier Wednesday, a coast guard spokesperson said the search would continue “as long as there’s a chance of survival” and that “we remain hopeful that we’ll be able to find him alive.”

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Stewart is known for his documentaries, which include Revolution, and his memoir, Save the Humans.

A global wildlife photographer, he has devoted his career to warning the world about threats facing sharks and other ocean life, as well as humanity.

His 2006 documentary Sharkwater, which debuted at the Toronto International Film Festival, became an international hit and prompted people around the world to lobby their governments for bans on shark finning.

Stewart said he and his colleagues risked their lives to make the film, visiting a Costa Rican warehouse that trafficked in illegal shark fin and confronting poachers on the high seas.

Stewart shot Revolution, which was released in 2012, in 15 countries over four years.

“This century we’re facing some pretty catastrophic consequences of our actions,” he said in a 2012 interview with The Canadian Press.

“We’re facing a world by 2050 that has no fish, no reefs, no rainforest and nine billion people on a planet that already can’t sustain seven billion people. So it’s going to be a really dramatic century unless we do something about it.”

In 2013, Stewart told the Star’s Greg Quill that oceans are in dire shape.

“Because they can no longer absorb and convert the hydrocarbons humans are producing, our oceans are 30 per cent more acidic than they were 100 years ago,” Stewart said. “Forty per cent of phytoplankton, which gives us half the oxygen we breathe, is gone.

“Four of the last five major extinctions that wiped most life off the planet were caused by ocean acidification — and we’re facing extinction now.”

“I never had faith in humanity,” Stewart told Quill. “I love animals, and I hate what humans do to them. That’s why I made Sharkwater. But watching people go into battle to save sharks as a result of seeing my movie has turned me around. I believe enlightened humanity can make a difference.”

Stewart attended Toronto’s Crescent School from grades 7 to 9 and is considered among its “distinguished alumni,” according to a statement released by the school.

He has since returned to speak to students about his work in marine conservation, the statement noted, adding: “Our thoughts are with Rob’s family and we share their hope for his safe return.”

Stewart also attended Lawrence Park Collegiate in North Toronto and earned his bachelor of science in biology from Western University in London, Ont. He also studied with Canadian Field Studies, taking courses in Jamaica and Kenya.

His parents, Sandra and Brian, are co-CEOS of a media group that publishes magazines distributed in movie theatres.

Stewart has credited them with broadening his contacts in the film industry, as well as being the executive producers and financiers for Sharkwater and Revolution.