“After almost 100 years on the planet, I now understand the most important place on Earth is not on land, but at sea,” Attenborough says in the trailer for new documentary Ocean with David Attenborough.

The documentary premiered this week in London with the King in attendance ahead of a major United Nations ocean conference in France in June.

Attenborough, who turns 99 on May 8, recalls his first scuba dive on the Great Barrier Reef in 1957.

“I was so taken aback by the spectacle before me, I forgot – momentarily – to breathe,” he said,

Since then, he noted there has been a catastrophic decline in life in the world’s oceans.

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“We are almost out of time,” he warns.

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Attenborough first dived Australia’s Great Barrier Reef in 1957. Image: BBC

The new documentary includes dramatic footage of the devastation fishing practices such as bottom trawling wreak on the seabed and its wildlife, as well as highlighting the wonder of natural habitats – from kelp forests to coral reefs – and the need to protect them.

In one sequence, the camera follows a bottom trawl, where nets are dragged with a metal beam across the seabed to catch fish, as it travels over the ocean floor, destroying the habitat, stirring up silt and scooping up species.

It also shows before and after footage of scallop dredging off Scotland’s Isle of Arran to highlight the damage it causes, with Attenborough warning that bottom trawling is still allowed in marine protected areas worldwide, and subsidised by governments.

Elsewhere, the film documents the damage being done to coral reefs by climate change warming the oceans and causing mass bleaching events, followed by a growth of algae on the corals which kills them off.

It also shows factory ships catching krill – a key food source for Antarctic wildlife from penguins to whales – on an industrial scale for fish farming, health supplements and pet food.

Attenborough spent time with the King at the movie’s premiere. Image: AAP

But Attenborough also offers a message of hope that, given the chance, the oceans can recover and provide food, store carbon dioxide in seabeds, kelp and seagrass, and allow wildlife to thrive.

“The ocean can recover faster than we thought possible,” he says in the film.

“If we just let nature take its course, the sea will save itself.”

The film includes footage of kelp forests off California where fishing has been fully prevented, which have recovered to healthy ecosystems and boosting lobster populations that spill over into the wider sea where catches have increased, and the world’s largest marine protected area off Hawaii.

Attenborough points to the reversal of fortunes for the world’s whales after an international whaling ban was secured and sites where coral recovers after bleaching where fishing is banned, as fish eat off the algae and allow the ecosystem to regrow.

And he concludes: “If we save the sea, we save our world.”