Diving into the world's aquarium

It seemed to be happening in slow motion. I was watching my dive- master who looked as if he was about to be mugged - by a huge bull sea lion. This monster was swerving towards him, diving down towards his head, his malign features contorted with rage that we had dared venture near his territory, his females and newborn seal pups. The sea lion let out an almighty bellow and our dive-master, realising that he was about to be pounded into the seabed, moved away - as fast as his fins would carry him.

We are diving Los Islotes, one of the most famous dive-sites of the Sea of Cortez, the strip of sea which runs between mainland Mexico and the Baja Californian peninsula. Los Islotes (literally, the little islands) consist of small pinnacles of rock joined together by a perfect archway.

The rocks are home to hundreds of Californian sea lions basking in the sun, sweet playful new pups, laid-back looking females, and at a ratio of roughly one to ten, huge don't-mess-with-me males. And they are making an almighty noise, bellowing, roaring, braying, while seagulls screech past overhead.

This is a wonderful dive site. The highlight of the day is when the female sea lions escape from their male captors and come to play. Relishing their freedom, they frolic with the divers, mimicking our actions as we all somersault. With their underwater acrobatic show over, they nip back to their rocks and leave us longing for more.

The alluring Sea of Cortez is still waiting to be discovered by many divers, despite being given top billing by none other than Jacques Cousteau, who described it as "the world's aquarium". Basing myself in La Paz, in Southern Baja, I found it a place that avoids the commercialism of the Caribbean and the crowds of the Red Sea - but offers exceptional diving because of waters so rich in nutrients that they groan with marine life.

Whales, whale sharks, mantas, hammerhead sharks and deep-sea fish such as marlins and sailfish, are all to be found here. What you see depends on luck - and the time of year - but even if you don't encounter any of these marine creatures you can content yourself with a myriad of other smaller species, some indigenous and many with tropical origins, even though only 50km of Baja is south of the Tropic of Cancer.

I was surprised to discover that coral reefs do not grow in waters as nutrient-rich as these, but learnt to appreciate the more subtle colours offered by the rocks and boulders that line the seabed, home to brightly hued starfish, eels, nudibranchs and Pacific seahorses.

One reason that the diving industry has been slow to take off here is that until recently, much of Baja, which is mostly desert, was relatively inaccessible. The distances to the dive-sites from La Paz are long. The pioneers of diving in this area are the English and Italian duo James Curtiss and Andrea Tomba who own and run the Cortez Club. When they started their operation in January 1996, they employed small, fast boats to overcome the problem of distance - with the added advantage for the diver that you are always in small groups.

During the boat-rides we enjoy the stunning topography of the region. Calm azure waters are edged with bays of perfect white sand and dotted with deserted islands; mountains studded with cacti recede into the distance and delicate white clouds are etched into a limpid blue sky. Pelicans perch on the rocks near the sea, a strange jumping manta called a mobulla leaps from the sea from time to time, and on one trip we saw several blue whales spouting only a few metres from our boat.

Unfortunately, it was the wrong time of year to see the grey whales that migrate through the waters of Baja from December until March. Although no diving is allowed around March, tourists can watch from boats as the whales give birth. I heard tales of tourists sobbing with emotion as mother whales bring their calves up to the boat to be stroked.

Another major attraction for people who dive the Sea of Cortez is to see schooling hammerhead sharks. These freaky-looking, yet shy creatures congregate in their hundreds at certain times of year at a spot known as El Bajo, consisting of three underwater peaks.

A sighting of the hammerheads is by no means guaranteed, but there are sites other than El Bajo. The first of these, on the north-western seamount, is not for the faint-hearted. It is quite deep for your average sports diver (we went down to about 38 metres) and there can be strong currents. Descending one by one down the anchor line to the top of the seamount, I noticed one of the densest concentrations of fish I have ever seen, schools of tuna and jacks mingling with pretty little damsel and angelfish.

The second dive on the central seamount is home to a staggering number of green moray eels, which writhe and snarl, twisting like green ribbons, poking out of every rock and crevice on the seabed like a watery vision of purgatory.

Stunning though El Bajo is, my favourite spot became the dive-sites around the Island of Cerralvo. Being further south, the water temperature was warmer than at other sites and the visibility exceptional. It also didn't seem prone to as many startling thermoclines (when different currents cause sudden changes in water temperatures). It was wonderfully remote and, despite a few currents, the diving was easy and relaxing. The density of marine life and concentration of tropical species surpassed even El Bajo. At one point I hung in the water while fish teemed above me, below me and on every side - I felt as if I were hallucinating.

And was I terribly disappointed to miss the hammerhead? Not at all. Now I have an excuse to go back.

DIVING IN THE SEA OF CORTEZ

GETTING THERE

To get to La Paz you will have to fly to Los Angeles or San Francisco, stopping overnight, and then take Alaskan Air or Aero California to La Paz airport. Virgin Atlantic offers two flights daily to Los Angeles (tel: 01293 747747) from pounds 235.

DIVING

Deborah Keeping's diving holiday was organised courtesy of Kuoni Worldwide Diving Holidays (tel: 01306 744345). Prices start from pounds 739 per person, including return flights, transfers and seven nights' accommodation at La Concha Beach Resort, plus one night in Los Angeles. Dive packages at the Cortez Club start from pounds 59 for a one-day, two-dive package, and pounds 259 for a five-day, 10-dive package. The Cortez Club offers whale watching expeditions between January and March for pounds 100.