Ask the friends who dived with late filmmaker and eco activist Rob Stewart what most scares them in the water, and the last thing on their minds will be sharks.

What’s perhaps more telling is what actually does scare those who helped make Stewart’s passionate, posthumous call to action Sharkwater Extinction. And many of those terrifying moments are caught in intense scenes on-screen.

For Brock Cahill, a close friend of Stewart’s and founder of the nonprofit SeaChange Agency, his most harrowing dive came during the film’s secret nighttime shoot of a massive drift net. Risking the current pulling him and Stewart into the net, they dived—deep—moving along it into the dark, their lights illuminating the giant predators tangled dead in its lines.

In Sharkwater Extinction, Stewart and crew travel from Costa Rica to Cape Verde to track an underground fishing industry that’s killing tens of millions of sharks per year. But they found this nightmare right off the California coast.

“You see this humongous wall of death. It feels like a ghost that’s in the water, like a living entity. I was shocked—I’m a California boy,” Cahill remembers, still shaken by the sight, as he talks to the Straight from his home in L.A. “But we had to do it. If people don’t see the harm that this is causing, they won’t understand what’s happening. Nobody had done this before. It was one of the most nerve-racking dives we’ve done in our lives.”

When they resurface, an altercation with a fishing boat makes things even more tense. “In our own back yard, there were warning shots fired at us,” Cahill recalls.

Exposing the damage done by drift nets has had an effect even before Sharkwater Extinction opens wide this week, with California passing a law within the last two weeks to phase them out, a goal Cahill’s charity has been aiming for for more than six years.

Bringing about change was something Stewart, who died in a tragic diving accident while filming Sharkwater Extinction, had a gift for. Trained in biology and zoology, he turned his lifelong love of both sharks and the water into his cause, shooting 2006’s Sharkwater in his 20s and raising his voice against finning around the globe. For the new follow-up film, Cahill says the key was always to show sharks as creatures of wonder, instead of Jaws-like monsters.

Veteran ocean and wildlife photographer Andy Casagrande says Stewart wanted to make Sharkwater Extinction, despite its moments of ugliness (tankers piled high with frozen blue-shark carcasses, or thousands of dismembered fins laid out in a secret warehouse), as beautiful as possible.

“He knew that images speak louder than words, and if kids see beautiful images of sharks it will have a bigger impact,” the long-time National Geographic and Shark Week photographer tells the Straight from his home in Florida. Over his career, Casagrande, too, has learned to love the creatures.

“I’ve been lucky enough to shoot polar bears and lions and cobras and jellyfish, all these dangerous predators, and countless sharks on all the continents, and they continue to be the most polite predator,” he asserts. “I can’t go into a pride of lions feeding, but I can go into a bunch of sharks feeding. I’d swim up to a great white any day without a cage. But a lion or a polar bear? That’s a different reality.”

Casagrande was on hand for one of the film’s ugliest moments, however. He accompanied Stewart on the boat of Mark the Shark, a fishing guide who openly hauls in hammerheads and other endangered species for paying clients. Casagrande points to that day as a lasting reminder of Stewart’s open, nonconfrontational approach to gathering information.

“I really just wanted to throw him overboard and feed him to the sharks, but Rob said, ‘Let’s learn about this guy and get into his head. It won’t help to get aggressive with him,’ ” Casagrande relates. “Sharks were getting killed in front of him, but Rob was in director mode and he knew he had to cover this.”

Both colleagues recall Stewart’s drive to instill hope even in the face of carnage in our oceans. And both miss him terribly, making the task of doing publicity for the film difficult, while reinforcing the sense that it’s essential to continuing his legacy.

After Stewart’s last dive off the coast of Florida, on January 31, 2017, when his rebreather went awry and he died of drowning due to hypoxia, the footage shot that day was confiscated for the ensuing investigation. Casagrande remembers getting the equipment back, with the memory card still in it, turning it on, and seeing the first clip: his late friend looking straight at the lens, waving, giving the peace sign, and flashing his signature smile right at them.

“He’s literally connecting with us from the beyond,” the cameraman marvels. “Anyone who knew Rob knows he had an uncanny way of connecting with people.”

His death, he says, was definitely a wake-up call for him and others who dive and film. “The average person would think the sharks are the most dangerous part of what we do. The reality is it’s everything else: the boats, the scuba-diving equipment.”

For his part, Cahill, who was on the boat for Stewart’s last dive, calls doing interviews about his beloved pal excruciating. But he gets through by remembering the amazing moments they had on this shoot—including a near-miraculous occasion at sunset off the Bahamas’ remote Cat Island, where both men were free-diving for the first time with oceanic whitetips—a species Jacques Cousteau, Cahill recalls, once called the most dangerous fish in the sea.

“They were some of the coolest creatures I’ve ever seen,” he recalls. “They’re a little cheeky and smart; they play with you and sneak up on you. One female had a little crush on me—she was all up in my grill.

“And all of a sudden the light changes, the sun starts to go down, and there’s this pink-golden light and the seabirds start coming down to feed on the surface,” he continues. “And Rob and I are sitting there with our cameras, and we did a 50-50 shot where you’re half out of the water and half in. And we got this incredible shot of this oceanic [whitetip] drifting by and the seabirds at the surface. That’s one of those memories that are just burned into my skull. That and Rob’s massive, huge smile.”