Diving ambition: Aspiring Jacques Cousteaus need a solid grounding in theory and practice to meet the challenge of the open sea. Claire Gervat reports

I am sitting under 10ft of water in a swimming-pool in Chiswick, breathing air from a large cylinder attached to my back. In front of me, a man is making hand signals. He looks as if he's playing aquatic charades; in fact, he is telling me to take my air supply out of my mouth on purpose, and calmly put it back in again without drowning. After that, I have to take my mask off, put it back on and clear the water out of it - without holding my nose.

It all seems a long way from the world of Jacques Cousteau, yet these are skills that can make the difference between life and death. The instructor who taught me to dive in Australia wore a T-shirt that read 'Do you remember when sex was safe and diving was dangerous?' There's some truth in that; diving is a lot safer than it was, but only if you do it right.

Beginners follow a three-part course. The first section consists of theory: classroom lessons about basic principles and safety tips. Never hold your breath is one of them; the air in your lungs expands as you rise to the surface and could make them burst. Nor is diving something you do to shake off a hangover; you need to be fit to scuba and regular medical check-ups are essential.

After that, come the practical sessions in the pool, where you get to practise what has been preached, and learn to deal with every potential problem. The surroundings may be prosaic, but the experience never is. Almost everyone who has ever learnt to dive can remember the ecstacy, almost religious in its intensity, of that first confident underwater breath.

Nevertheless, after five pool sessions, the view begins to pall. You find yourself watching 'Le Grand Bleu' obsessively, dreaming of dolphins. Now you are ready for the final step - the regulatory four dives at sea that you need to become a certified open-water diver. If you plan to do these off the coast of England, the school will tailor the training towards the local conditions which are necessarily more demanding than tropical areas. Cold-water phobics can complete the course abroad.

My first sea dives happened on part of the Great Barrier Reef, in warm water and with beautiful marine flora and fauna to look at. But the British are not short of their own good diving sites.

According to Stephen Mawle, an instructor at Ocean Leisure, British divers are, in some respects, very lucky, with 'more wrecks per mile of coastline than any other country'. The diving may be challenging, but the rewards for the bold can be great. 'We have shallow seas with spectacular drop-offs and an abundance of marine life. If you're prepared to work at it, you can dive with seals, porpoises and even killer whales.'

Most people have, at some time, longed to do just that. But not everyone is cut out for it. If you're not sure about life under the ocean waves, you can always do a supervised 'try dive' in the pool for 45 minutes, long enough to get you hooked.

And once you are ensnared? While you're still a novice it's easy to see where the thrills come from - just being in a new and strange environment, hoping that nothing will go wrong. With experience, the adrenalin buzz diminishes. But if you miss the tension at that stage, you could always try swimming with the sharks.

The full beginners' course with sea dives costs pounds 335, including the use of scuba equipment. Ocean Leisure at 201 Chiswick High Road, London W4 (081-742 8585). For outside London details contact PADI UK in Bristol (0272 711717)

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