In the 2006 documentary Sharkwater, Rob Stewart, the film's writer, director, producer, narrator and star, did things that few others ever attempt. He went scuba diving with sharks of numerous species, often stroking and even hugging them. He also took a lengthy free dive – no oxygen tank, just holding his breath – with sharks.

In another daring move, Mr. Stewart climbed a building to film illegally obtained shark fins drying on a roof. His boat was chased out of Costa Rican waters by men with guns. He contracted a flesh-eating disease after sustaining cuts on his body from diving.

"Rob knew that in order to get people to watch his films he needed [to include] a lot of action in them," says Dustin Titus, a friend and colleague who helped market some of Mr. Stewart's films. (After the filming for Sharkwater was over, Mr. Stewart was sick for months, suffering from dengue fever, West Nile virus and tuberculosis.)

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The Toronto-born photographer, filmmaker and environmental activist was 37 when he died on Jan. 31 in the waters off the Florida Keys, halfway through shooting his new film, Sharkwater: Extinction.

He regularly pushed limits to get gorgeous, heart-wrenching footage for his documentaries and combined the images with plainly stated facts. He aimed to dispel myths and show how sharks' plight has an impact on human life.

"He had this incredible gift of being able to show the beauty of the world," says Sarika Cullis-Suzuki, a friend who is a marine biologist, activist and daughter of David Suzuki. "We too often focus on the battles and what was lost; he showed us what we still have."

His approach worked: Sharkwater won more than 40 awards around the world and will air on Netflix later this month. The film and his ceaseless advocacy resulted in numerous bans on shark fishing and shark-fin soup around the world. (He sent China a copy of the film and it aired on state TV. Consumption of shark fins dropped afterward – by half in Hong Kong alone.)

He followed up with 2012's Revolution. It took him four years to film this wider look at the environment and activism. In a 2012 interview about it and his book Save the Humans (Random House Canada), Mr. Stewart told the Canadian Press, "We're facing a world by 2050 that has no fish, no reefs, no rain forest, 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."

Revolution won 19 awards. In 2015, he released The Fight for Bala, a film about the at-risk Bala Falls in Muskoka.

In addition to his films, he co-founded the non-profit United Conservationists, which funds the Fin Free campaign, nature reserves and other environmental projects around the world. He was known worldwide for his public speaking and his skills as a diver, and became friends with celebrities including billionaire Richard Branson and actor Adrian Grenier.

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Young, handsome and svelte, Mr. Stewart played well to the camera. He had an engaging surfer-dude drawl – you would never know that he stuttered as a kid and trained himself out of it over many years.

Julie Andersen, a United Conservationists co-founder who collaborated on films with Mr. Stewart, marvelled at how he was able to remain perfectly dressed and coiffed even while filming in a hot, damp Madagascar jungle.

"We're going to make the environmental movement cool," he once told her.

To him, conservation was far more than a fad, though. When Mr. Titus first went to Mr. Stewart's apartment around 2006, he was surprised to see wall-to-wall books: serious literature and complex biology textbooks that he had clearly read. "Rob looks cool all the time but he's actually super nerdy."

When Ms. Cullis-Suzuki first met him, at an event, she asked what impact Sharkwater had made on the world. Without a pause, he began listing all the regions where shark fishing had become illegal. "He knew all the stats off the top of his head," she says.

Robert Brian Stewart was born on Dec. 28, 1979, in Toronto, to Brian Stewart and Sandra Campbell, entrepreneurs who own and run Tribute Entertainment Media Group. Obsessed with animals from a young age, Rob got his first goldfish around four.

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Visiting the fish store was a weekly routine and staff there soon began calling him with news of new arrivals. The boy later got a monitor lizard and a boa constrictor, which he named Mali. "His bedroom was like a menagerie," his father says.

On family vacations in the Caribbean, "we'd still be unpacking and he'd be in the water, looking for creatures and talking to the locals," his father recalls. Rob saw his first shark at the age of eight, and fell in love. When he was 13, he insisted his entire family, including elder sister Alexandra, get scuba diving certifications. He got his first underwater camera at the age of 14, learned to free dive at a young age and got his scuba instructor certification at 18.

At the family cottage, Rob busied himself catching frogs and other wildlife. He'd often swim alone to a nearby island, despite his parents' fear that boats would hit him. "I'll just dive to the bottom until they go by," he replied.

Rob excelled at his studies and went to the private all-boys Crescent School starting in Grade 7. He met Mr. MacLeod that first day; they were the only two with long hair. They'd hang out at Mr. Stewart's house, among the many fish tanks and animal books.

While Mr. MacLeod would look at the books' pictures and the captions, his buddy seemed to have all the content memorized. "He was like a human encyclopedia regarding anything to do with animals and the ocean," says Mr. MacLeod, who helped his friend with films and activism years later. "He was obsessed with animals, and I was, too, but he was [at the] next level."

Rob eventually told his parents that school was "really boring with just guys." They said he could go to public high school at Lawrence Park Collegiate if he stayed on the honour roll. Several of his buddies changed schools with him. There, he played on the rugby team and excelled academically.

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He went on to study biology at Western University, taking lots of math because, he told his parents, it was a "bird course." He took advantage of special exchange programs that took him to Kenya and Jamaica.

In Kenya, students went out to collect wildlife and share it with the class. Others found snails and crabs but they gathered around Mr. Stewart's container, teachers included, knowing he'd have something good. He opened it to reveal a black mamba, one of the world's deadliest venomous snakes. Everyone leaped back, terrified, while Rob picked up the snake, saying, "Check it out, guys!"

After graduation, he did photography for the Canadian Wildlife Federation's magazine, which was published at the time by his parents' company, and did freelance work as well. That work took him all around the world.

He wanted to have more of an impact, though, so he bought a video camera and devoured a book on how to make movies, which a girlfriend gave him. He began shooting Sharkwater in 2002, at first flying on points and using his own money, plus some from his parents, then eventually receiving some tax credits and distribution support.

Thus began a whirlwind career of travel, creating films, speaking, working on projects with others, and constant learning. Mr. Stewart accomplished a great deal quickly. "Rob's parents are very successful business people [and] his sister went to Harvard. But Rob took that same work ethic and intelligence and drive and applied it to the planet," Mr. MacLeod says.

Those close to him marvel that, despite everything he knew and saw – hundreds of sharks slaughtered, dying reefs, the shark-fin mafia wielding weapons – Mr. Stewart remained an optimist. Ms. Andersen says: "He saw some pretty gnarly stuff. But he had incredible faith in mankind and our ability to change. He made you believe anything was possible."

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Rob Stewart leaves his parents; sister; brother-in-law, Roger Rudisuli; and two nephews.

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