When I was a child, one of my favourite television programmes was a cartoon called Marine Boy. I would watch, amazed, as the amphib-ious hero chewed his Oxygum and plunged maskless into a multi-coloured underwater world to save mermaids in distress and avert ocean catastrophes. I dreamed that one day I too would swim with the fishes.

When I was older, snorkelling in the clear, warm waters of the Mediterranean, I saw glimpses of sea creatures and plant life that made me want to do more than skim the surface, to go deeper - to scuba dive.

So here I am, on my 34th birthday, sitting on the edge of a rigid inflatable boat in Portland Bay, dressed in wet suit, mask and fins, festooned with tubes, looking like Donald Duck on life support and about to make my first sea dive.

It’s the third day of my four-day British Sub Aqua Club (BSAC) ocean diver course. The other people on the course are my friend Ged, another long-time scuba wannabe, and Matthew, a local who has bought all the gear and decided he’d better learn how to use it.

The green-grey waters don’t look too inviting, and as the boat lurches on the swell I realise that a fry-up for breakfast was my first mistake of the day.

I hope it’s my last, because errors can be costly in scuba diving. Two days of lectures in the classroom have taught us all about that. Go down too deep and you’re liable to get nitrogen narcosis. Come up too quickly and you can suffer decompression sickness, otherwise known as the bends. Hold your breath on the ascent and your lungs might explode.

If you make a mistake you enter “the incident pit”, a scary place where problems multiply and no one gets out alive. “You don’t want to go there, ” warns our instructor Tim.

Repeated drills in the pool have (we hope) taught us what to do should something go wrong underwater, but before taking the plunge there’s one final safety ritual to observe.

I run through a buddy check with my diving partner Matthew, making sure our bouyancy compensators (a kind of inflatable lifejacket) are working, indicating the quick-release points on our kit, and double-checking our air supply.

Ian, owner of the Old Harbour Dive School in Weymouth and veteran of several thousand underwater sorties, is our instructor on the dive. So, with a backward roll over the side, a thumbs-down signal to submerge from him and an okay from us, we begin our descent.

It’s nothing like the movies. All that slow-motion, sub-aqua action looks great in Cinemascope, but scuba diving in British waters bears more resemblance to a grainy old black-and-white film. Visibility is about three or four metres, and the shimmer of light on the surface soon disappears as we sink along the anchor line, dumping air from our jackets to aid our descent and equalising the pressure in our ears by holding our noses and blowing.

Down on the sea bed, the tank strapped to my back and the lead weights round my waist don’t seem so heavy and the wet suit is warming up nicely. It’s murky but cosy, like sitting in a very deep bowl of pea soup.

Ian signals for us to practise our safety drills. We take our regulators (the rubber contraption the air comes through) out of our mouths, wait a few seconds, and put them back. Easy.

The problems start with mask clearing. I breezed through it in the pool, but taking off my mask in deep, cold water seems like a very bad idea indeed. When I do, mild panic sets in, I breathe up my nose (even worse idea) and splutter through the drill.

It’s a great relief to reach the surface (“Like coming back to life,” says Matthew, who has the same mask-clearing phobia) and, after two more shallow dives and a short boat ride, we are back on dry land. But before we can relax we have to re-sit a test on the theory we have learnt. There’s a lot of maths, working out air requirements, pressures, dive times and decompression codes. Having just failed to get the minimum 80 per cent mark the day before, this time we all pass.

In the pub, Ian assures us that learning in this country is the best preparation you can have. Because conditions here are so difficult, dive resorts with warm, clear water will be easy for us. One of his favourite sayings is: “You pay for the course. You earn the qualification.” It certainly feels like it. I never thought diving would be so mentally and physically exhausting. That night I sleep for 11 hours.

On the fourth day we go looking for wrecks, a pursuit that makes this stretch of the Dorset coast a Mecca for divers.

My dive buddy and instructor is Terry, a former murder squad detective from Hackney who retired to Weymouth for the diving. “I’d seen enough dead bodies, ” he says. It sounds vaguely reassuring.

We are going to see HMS Hood, a 100-year-old battleship that lies partially upturned across the harbour entrance at a depth of 20 metres - the height of a five-storey building and the deepest I have been.

After four days of trying, I have at last managed to achieve neutral bouyancy, that perfect state of weightlessness that divers aspire to. It’s like walking on air.

The hulk of the ship rises out of the gloom and, as we fin along its rusty frame, past gun turrets embedded in the sand, striped fish and white mushroom-like coral, I finally feel like a diver.

‘TES’ readers can learn to dive at The Old Harbour Dive School in Weymouth throughout the summer at a specially reduced rate of Pounds 245. The fee includes five open water dives, equipment hire, 2:1 pupil to instructor training, and an information pack. Contact Ian Fuller on 01305 761666 * Diving in

There are two main ways of becoming a qualified diver. The Professional Association of Dive Instructors (PADI) open water qualification is less demanding than the British Sub Aqua Club ocean diver course, which incorporates rescue and navigation skills.

Both certificates are internationally recognised and will enable you to hire equipment and dive with someone of your own ability or above anywhere in the world to a depth of 20 metres.

As well as being the sport’s governing body in the UK, BSAC also runs dive clubs which allow members to practise their skills and organise trips abroad. Both organisations run more advanced courses.

Tel: BSAC 0151350 6200; PADI 0117 971 1717.