IF television is a window on the world, then Jacques Cousteau opened that window on to one of the greatest and largely unknown frontiers on Earth - the oceans. In the 1960s and 1970s he took viewers on an unforgettable armchair adventure into the unexplored deep, to a place below the waves that was tranquil, exquisite and serene.

He was one of the first great pioneers of television nature programmes and he tackled one of the most difficult habitats to explore. He gave us some of the first pictures of the oceans and its inhabitants, creatively packaged in a documentary.

Cousteau, who died this week aged 87, knew that television was the perfect medium for him, and he soon became one of the French men most famous outside France.

He once said: "With television you know that on one evening 35 to 40 million people are going to see dolphins.

His films were novel and exciting, but there was usually a gentle and often sentimental reminder that, this underwater world was precious and vulnerable.

He was a popularist, showman, filmmaker, inventor and engineer passionate about the oceans: a striking man with his weather worn seacliff face, red woolly hat and binoculars slung around his neck, an icon for explorers.

Most memorable of all were his adventures on board his ship, the Calypso, a converted British naval mine sweeper. Always busily plotting and planning the next venture, he challenged his team to photograph the unphotographed, to explore the unexplored - the vast enchanting oceans right down to the tiniest unknown creature.

The impact of his films was immense, not only on an audience hungry for adventure and wildlife, but on those who wanted to follow in his footsteps.

TO me, as a boy growing up in Dublin in the late 1960s and early 1970s, his television series, The Undersea World of Jacques Cousteau, was an escape to another but real world of adventure and exploration. And he sowed the seed in my mind, and in those of many others, of the kind of job I wanted to do in the future.

To make the pioneering films he produced, new undersea equipment needed to be designed so that the camera operator could be as flexible and flee swimming as possible. Cousteau played a vital part in the invention of two of these before he embarked on his films: the aqua lung with Emile Gagnan in 1943 and the underwater camera with Andre Laban in 1950.

Shortly afterwards Noel Guinness, the Anglo Irish businessman, gave him the ship that was to become the famous Calypso, and so he began his adventures.

He went to the Red Sea and made his first colour film at a depth of 150 feet. In 1952 he looked for sponsorship and was backed by many research institutes, including the American National Geographic Society to go on a four year, round the world trip.

Over four decades he filmed more than 100 documentaries from the Calypso, including Silent World in 1956, a film about his undersea explorations, which won the Cannes Palme d'Or.

This pioneering real adventure documentary was beautifully crafted. It stunned cinemagoers and in 1957 won him an Oscar. In 1964 he won another Oscar with World Without Sun.

His greatest impact was to come in 1966 with his first important television documentary, called The World of Jacques Yves Cousteau.

This was followed in 1968 by the famous television series, The Undersea World of Jacques Couteau, a multi million dollar venture backed by an American television company. It was phenomenally successful and ran for eight years, eventually being shown all over the world.

This was the dawn of the more technically - as well as creatively - crafted wildlife films that soon had audiences glued to their TV screen. With his authoritative but passionate voice, Jacques Cousteau showed us the real wonders of nature.

His son, JeanMichel, said of his father: "The mystery of life down to humblest forms was to him a source of infinite curiosity of absolute wonderment and serenity."

Just like Cousteau, today's wild life programme makers still need to develop new filming techniques to bring better and more stunning images to our screens, to help in our understanding of the natural world. His legacy is that he pioneered undersea photographic techniques that are now standard to all underwater cameramen.

Cousteau was considered arrogant by many. He often offended and fell out with many colleagues - including his son, JeanMichel, when he tried to set up a Cousteau holiday resort in the Pacific. He was also a brilliant manipulator and persuader.

On location, he ordered and commandeered ships, boats and people, sometimes without paying them, and never gave them credit for their work. His abuse of local populations and faking of underwater shots was greatly rumoured, but he generally ignored the rumours.

By the mid 1970s his reputation as a scientist and publicist was being questioned. He was accused of using the good name of science to justify films and expeditions, and trying to give simple solutions to environmental problems.

The BBC made a programme called The Credible Captain Coils tea it. In it, Cousteau had to defend himself under questioning before an invited studio audience of environmentalists and scientists. He did so by saying he was never a scientist but an engineer and impresario, a man who got things done and gave others a chance to discover truth and to change the world.

In the end, he tried to show all he was neither self deceiving nor a hypocrite.

AS HE opened the under sea world to us as viewers he changed the way we perceived the world around us. Man was now walking on the moon and our perception of the planet Earth was changing for ever. There was only one Earth and most of it was ocean.

As time went on, he became more acutely aware that the oceans he filmed were being more heavily polluted and he became more passionate about its conservation. He changed from wildlife filmmaker to an articulate conscience on environmental issues.

In 1973 he declared he was against spear fishing. He led a crusade against pollution, fought to save Antarctica from exploitation, pioneered wind and solar power, built underwater accommodation for divers, and set up the Cousteau Society in the US, a centre of research and education.

The future of mankind was his main theme: he sponsored the Bill of Rights for Future Generations at the UN and became a star at the Rio Earth Summit in 1992.

He took a stance against President Chirac's resumption of nuclear testing in 1995 and warned of the dangers of decaying nuclear installations in the former USSR.

But for many it is for his television programmes that he will be best remembered. His films are immortal. Not only are they unique and beautiful but they also touched the audience's sense of wonderment at the oceans when we knew little about them.

As a result, he influenced a whole generation of people into taking up scuba diving. Today it has become one of the fastest growing sports around the world.

He inspired students to study marine biology and he also gave credos to the environmental movement and influenced another generation on the importance of conserving our fragile living planet.

Last but not least, he was an inspiration to many other wildlife filmmakers.