The English Channel is one of the world's busiest shipping lanes, used by hundreds of cargo vessels and ferries every day.
But although swimming the Channel is a remarkable feat of endurance - fewer people have managed it than have climbed Mount Everest - mishaps during cross-Channel swims are rare.
Both the Channel Swimming Association and the Channel Swimming and Piloting Federation (CS&PF), which authorise and support attempts, require medicals signed by a doctor and minimum levels of training.
The first person to swim the Channel observed and unassisted was Captain Matthew Webb in 1875. It took him 21 hours and 45 minutes.
The first recorded unaided attempt to swim across to France was made by JB Johnson, from England, in 1872.
His attempt was abandoned after an hour and three minutes.
Australian Trent Grimsey holds the record for an unassisted solo crossing - six hours and 55 minutes.
Little Britain star David Walliams swam the Channel in 10 hours and 34 minutes in 2006, raising hundreds of thousands of pounds for charity in the process.
The shortest distance between England and France is 21 miles, but currents and the need to avoid shipping lanes mean swimmers may need to swim up to 30 miles to make it to the other side.
Swimmers face dangers not just from low temperatures, which range from 15C (59F) to 18C (64.4F), but also from jellyfish, sewage, seaweed, ships, adverse weather conditions and floating debris.