British endurance swimmer Lewis Pugh is to undertake seven swims in the Seven Seas to highlight the need for protected areas in oceans around the world.
He will be the first to undertake a long-distance swim in each of the classical Seven Seas, the Mediterranean, Adriatic, Aegean, Black Sea, Red Sea, Arabian Sea and the North Sea, ending with an 100km (62 miles) swim up the Thames to Parliament.
Mr Pugh, who will complete the seven swims this month, is backing calls by the United Nations for 10% of the world's seas - both around countries and on the high seas - to be declared marine protected areas by 2020 to safeguard fish and other wildlife.
Just 3% of the world's marine areas are protected, compared to around 13% of the world's land area.
Mr Pugh, who has previously undertaken swims at the North Pole and in a glacial lake in Everest to highlight rising global temperatures, said seas were threatened by pollution, overfishing and climate change.
"If we don't do something about it, we're going to be living in a world devoid of wonder and beauty, and frankly not sustainable. If we've achieved it on land, we can do it in the sea."
Marine protected areas are like national parks in the sea, with varying protections which could include measures such as preventing fishing, he said.
"If you care about the environment, about oceans, food security, these incredible animals that live in the seas, marine protected areas is a way forward.
"We rely entirely on our oceans to survive, it's not as if we have an option, we have to do this," he said.
He said the Seven Seas he would be swimming in, for distances ranging from 1km to 100km, were some of the most polluted and overfished in the world.
"The North, Aegean and Mediterranean Seas have been drastically overfished. Many coral reefs are in decline - through habitat destruction in the Red and severe bleaching in the Arabian Sea.
"The Black Sea is dreadfully polluted. And the rich wetlands of the Adriatic no longer provide a safe haven for thousands of migrating birds."
They would also present some particular challenges, he said. The swim in the Arabian Sea was originally due to be off the coast of Mumbai, but Indian authorities prevent swimming off the coast during the Monsoon, so it will take place off the coast of Oman.
In Monsoon season he has been told to expect a rough ride on the sea. "It'll be a tough swim," he said.
The final leg of his swimming odyssey will see him swim up the Thames from Southend, Essex, to Westminster to deliver a petition to Prime Minister David Cameron urging him to support marine protected areas.
Mr Pugh said the challenge with the Thames was that it was tidal, so he could only swim for six hours at a time, while the tide was coming in, before getting out and waiting for the outgoing tide, which it was impossible to swim against.
He also said he expected to get ill from the pollution in the Thames, as he had done when he swam the river from its source to the sea.
Mr Pugh, who is from Plymouth and has lived in South Africa, is backed in his campaign by high profile figures such as Prince Albert II of Monaco and Desmond Tutu, who joined the swimmer at his final training session to wish him well.
Mr Pugh and his team are also urging the public to support the campaign and put pressure on their governments to protect the seas, and people can even join the swimmer in the sea to swim parts of the swims with him.
The seven swims are as follows:
:: Mediterranean: Mr Pugh will begin his campaign in Monaco on August 7-9, with a 10km (6 miles) swim off Monte Carlo to honour the efforts by the nation to designate all of its seas as marine protected areas.
:: Adriatic Sea: The second swim of 15km (9 miles) will be off Zadar, on the Croatian coast, to highlight the near extinction of the slender-billed curlew which is hunted in the area on its migratory journey from Africa to northern Europe, and the loss of many other birds.
:: Aegean Sea: The 10km swim off Athens, Greece, will highlight the plight of the Mediterranean monk seal, whose once plentiful numbers have fallen to around 200 animals in Greece due to being shot and accidentally caught in fishing gear.
:: Black Sea: A 40km (25 miles) swim in the inland sea, which is very vulnerable to pollution from its shores, rivers and visiting ships, will highlight the need for greater protection through marine protected areas.
:: Red Sea: The 15km swim off the coast of Jordan, from Aqaba marine park to Tala Bay, will highlight the importance of mangroves which stabilise coastal areas, support coral reefs, provide nursery grounds for young fish, and timber and food for people.
:: Arabian Sea: The 10km swim off Rass Al Hadd, Oman, in the sea that spans from East Africa to west India will highlight the need to protect and restore coral reefs.
:: North Sea: The 100km (62 mile) swim up the Thames will highlight the loss of species such as the common skate which is now rarely seen in the North Sea, and urge the Government, which has announced some marine protected areas, to take more action to protect the skate from extinction.