Daybreak brings with it a drop in the wind – I am not sure what it is about this year but warmer weather and a lack of wind have been characteristic of our races. Still, our decision to head west proves beneficial as when the tide turns it pushes us in the direction of our heading to Cherbourg. It is slow progress but certainly the right decision. This passage, from Portsmouth to Cherbourg, is one of the furthest between England and France. The narrowest stretch being that between Dover and Calais and it is at this point that most people chose to cross – particularly those who swim the channel. Last year, 29 people achieved this feat and people continue to take up the challenge.

The two most famous swimmers must be Matthew Webb, the first person to do so, and David Walliams of Little Britain fame. Webb completed his goal on the second attempt in August 1875. Backed by three chase boats and smeared in porpoise oil, he set off into the ebb tide at a steady breaststroke. Despite stings from jellyfish and strong currents off Cap Gris Nez which prevented him reaching the shore for five hours, finally, after 21 hours and 45 minutes, Webb landed near Calais. His zig-zag course across the Channel was over 39 miles long. Walliams on the other hand completed the swim in 10 hours and 34 minutes by covering a 22 mile stretch of sea. Walliams’ achievement is all the more impressive as he managed to complete the passage by swimming just one additional mile over the distance (because of the tide). But it is not the distance between the coasts that interests me but the depth of the water.

The Strait of Dover at the eastern end of the Channel is the narrowest point, while its widest point lies between Lyme Bay and the Gulf of Saint Malo near the midpoint of the waterway. It is a relatively shallow stretch of water, with an average depth of about 120 m (390 ft) at its widest part, reducing to a depth of about 45 m (150 ft) between Dover and Calais.

I notice that the average depth over our track is about 60 m (180 ft) and I wonder what is beneath us. The bottom is probably very smooth as the land beneath the sea was originally dry land for most of the Pleistocene period (1.8 million to 10,000 years ago). It is thought to have been created between 450,000 and 180,000 years ago by two catastrophic glacial lake outburst floods caused by the breaching of the Weald-Artois Anticline, a ridge which held back a large proglacial lake in the Doggerland in the region, now submerged under the North Sea.

As a scuba diver who uses ‘normal’ air, I can go down to a depth of 40 metres but there are many people who use a combination of gases which allow them to go deeper. One of these people is an Edgware boy, Mark Ellyat. Mark holds the record for the world's deepest dive reaching 313m in 2004, 35 miles off the coast of Phuket in Thailand after a dive lasting a total of 6 hours and 36 minutes.

It is hard to imagine what it is like at that depth but those who have been almost that far describe it as lonely, like a trip to the moon. Below 200 metres surface light dies, with total darkness ensuing from 180 metres. Secondly, the water temperature drops to about 4 degrees centigrade. Mark’s book describes how his descent took just 12 minutes. He was checking the depth by the taped measurements on his plunge line with head-mounted lights. Although his dry suit kept the warmth in, he started to shake. He couldn't decide whether this was from the cold or from helium gas-induced tremors.

At 313 metres his self-planned dive schedule was running one minute over, and his constant mental safety checks got more and more complacent. At this point he looked down and saw the ghostly image of some kind of large hydroid. "I scanned left to right to check for any visual abnormalities and to check the distance more exactly of my jelly-like visitor," he says in the book. "The little checks I did told me that my concentration was sometimes stalling into a complacent mind-lock, and this set the narcosis alarm bells ringing."

This two-metre hydroid looked on course to hit his descent line. "I checked the depth on the line," he says. "It was deep enough for that day. I grabbed the line marker at 313 metres and headed for 249 metres, the first deeper stop. Mark shut his eyes and ascended hand over hand. The time spent at lower depths is crucial to the survival of a diver. The longer you stay, the more risk you run.

Then began the long wait while Mark's body accustomed itself to the changing pressure at such a depth, all the while obtaining necessary oxygen through long regulator hoses from the mother boat above. At this point, an accident that would have been comic in other circumstances occurred. His tongue almost got sucked through the regulator, causing the hose to collapse briefly.

During the stops, he ate fun-size Mars Bars and banana pieces while the support divers were having their lunch. At last, after 6 hours and 36 minutes under the Andaman Sea, he surfaced. "I had got down deeper than any solo diver before me," he says. "I had surfaced under my own strength and, more importantly, without decompression illness. Although I was exhausted, I was very happy."

Few people venture under the English Channel to that depth but many more choose to cross over it and it was not long on this latest voyage that I started to be asked questions about how we could do so in the future.