He was the French adventurer who plumbed the depths of the world’s oceans to introduce us to a magical and previously unseen universe under the sea. Commander Jacques-Yves Cousteau, the former naval officer turned inventor of the Aqualung and scuba equipment and then television explorer, became a hero to generations of children who were mesmerised by his adventures and groundbreaking films.

Now a new documentary explores his life and legacy, showing how more than half a century ago Cousteau sounded the alarm over the destruction of the oceans, which he saw as vital to the future of the human race.

While a young David Attenborough captivated viewers fascinated by nature on land, Cousteau, with his red beanie and weatherworn profile at the helm of the Calypso, was the old man of the sea, co-star to shoals of colourful fish, sharks and coral reefs in The Undersea World of Jacques Cousteau.

“Undersea film-making was extraordinarily rudimentary before Cousteau came along and revolutionised it. It’s hard to remember now, but this was a totally new world he showed us,” said Liz Garbus, the award-winning director of the new film about the oceanographer, Becoming Cousteau.

In the 1970s, after decades diving and exploring and at the height of his fame, Cousteau changed tack: instead of simply showing and sharing his exploits, he began worrying about and warning of the damage humanity was doing to the planet. Today, as world leaders gather in Glasgow to discuss the climate emergency, Cousteau’s fears for the future of the marine environment seem prescient but ignored. At the 1992 Earth Summit in Rio, the oceanographer – nicknamed “Captain Planet” – was received as a hero and was the only non-politician in the official portrait.

“Cousteau was warning about the dangers of climate change even before those two words were used together in a sentences, before we had this notion of global warming,” Garbus said. “He became an ardent environmentalist well before his time.”

Cousteau, who died at 87 in 1997, began his career in the navy training to be a pilot, an ambition cut short by a serious car accident when he was in his mid-20s. Part of his physical rehabilitation was swimming and Cousteau developed a love of diving, developing the first Aqualung, and after the second world war setting off to explore the world’s seas and oceans in the Calypso, a converted minesweeper.

The 90-minute National Geographic Documentary film, released in UK cinemas from 12 November, reveals how Cousteau aimed to become the John Ford or John Huston of the marine world, developing the first underwater cameras and inventing new ways of filming. For many it was the first time they had ever seen the wonders of the deep. But, short of money in the 1950s and wishing to keep the Calypso and his research afloat, Cousteau accepted funding from British Petroleum to help look for oil in the Gulf. Reflecting later on the environmental damage wreaked by offshore oil exploration, he would come to regret the decision. “I think I was naive … but we didn’t have a penny,” he says in the film.

Cousteau and his band of adventurers’ early interactions with marine life would be considered unacceptable today: footage shows them setting off dynamite charges to kill fish, “riding” turtles and, in the Oscar-winning 1956 documentary The Silent World, revelling in the killing of a shark that fought to the death.

Nor does the film gloss over his personal failings. Cousteau had many talents; being a family man, attentive father and faithful husband were not among them. His two sons, Jean-Michel and Philippe – the latter who worked with his father and who died in a plane crash in 1979 – were sent to boarding school while the workaholic Cousteau and his wife Simone sailed the oceans. He married his second wife, Francine, shortly after Simone died of cancer, by which time he already had two children with her.

After the death of his son, Cousteau’s films became darker, more pessimistic about the fate of humans, which he saw as inextricably linked to a disappearing underwater world.

The Oscar-nominated Garbus, who has been responsible for portraits of Nina Simone and Marilyn Monroe among others, said she had grown up on Cousteau’s extraordinary documentaries but realised when reading a story about him to her son that he had “receded into the woodwork, disappeared completely” from the public domain. “I went online and I couldn’t find anything much about him there either,” she said.

“I wanted to explore how he moved from his early start as a conquering adventurer to someone who was an ardent environmentalist ahead of his time,” Garbus said. “He changed from not just showing and sharing but protecting. In the 1970s he starts talking about having been diving for decades and observing reefs dying and species of fish that were plentiful disappearing. He was the most popular voice for conservation in the 1970s and then he just disappeared.”

Garbus said Cousteau’s enduring importance was as a “non-partisan figure uniting us around this issue”, a powerful role she believes nobody can fill today.

“His legacy is up to us. Is it a Cassandra story of someone warning of impending doom and the world not listening, or a story that we take action, but too late, but still some action?

“As Cousteau says, you will only protect what you love. He brought us closer to the undersea world and its creatures, and now we do love it and want to protect it and that’s thanks to him. That’s his legacy.”