Johannesburg - Endurance swimmer Lewis Pugh is the first person to swim the seven seas of the ancient world.
Pugh completed his final swim from Southend-on-Sea up to the Thames Barrier on Friday. It took him eight hours and 12 minutes.
The seven seas are the Mediterranean, Adriatic, Aegean, Black, Red, Arabian, and North.
“I've experienced some things I will never forget,” he said in a statement.
“And seen some things I wish I could erase from my memory, but which will haunt me for the rest of my days.”
Pugh is a ocean activist and the United Nation's environment programme's patron of the oceans.
He was born in the United Kingdom and when he was 10 his family emigrated to South Africa.
Pugh said he wished he could forget what he saw on the sea floor when he swam the Aegean. It was covered with litter such as tyres, plastic bags, bottles, cans, shoes, and clothes.
In the Arabian Sea, Pugh said he saw vast shoals of turtles, which was spectacular. However many other fish species meant to be there were nowhere to be seen.
“I never saw any fish bigger than the size of my hand, in any of the seven seas. The larger ones had all been fished out,” he said.
The Black Sea was full of jellyfish, which he said was not a good thing because they did not belong there and wrought havoc on an already imbalanced ecosystem.
Pugh said not during any of his swims, which started on August 9, did he see one shark.
“As I was about to jump in the water for the Red Sea swim I asked the boat's skipper whether I should keep a lookout for sharks.
“He told me not to worry, because the sharks have all been fished out.”
However, while swimming through a marine protect area in the Red Sea he said he saw the sea as it was meant to be “rich and colourful and teaming with abundant life”.
Pugh said there was a reason the swim ended in the North Sea at the Thames Barrier.
“It's a highly symbolic example of foresight and visionary design. When it was commissioned 30 years ago, its engineers had no idea how crucial it would be.”
The engineers thought it would be used two or three times a year, but last winter it was used 48 times, he said.
“Where would London be today without the Thames Barrier? In a word: underwater.”
Pugh said the world's waters were changing and the seas and oceans were in a state of crisis and needed to be protected.