(CNN) Having previously swum in the world's coldest waters wearing just his swimming trunks, for his latest challenge Lewis Pugh went to the opposite extreme.
When he became the first person to swim across the Red Sea last month -- a feat which took 16 days and saw him encounter crashing waves, busy shipping channels and extraordinary sea life -- Pugh toiled against what was by far the warmest ocean he's ever experienced.
As the sun beat down on his back and the water temperature sometimes crept above 30 degrees Celsius (86 degrees Farenheit), Pugh found himself battling exhaustion and dehydration -- even when he restricted himself to swimming at the coolest times of the day.
"It's a significant challenge," he tells CNN Sport, "and the challenge comes because one feels just so weak and lacking in energy."
Pugh, an endurance swimmer from the United Kingdom, is accustomed to tackling extreme conditions in some of the world's most remote oceans.
But the marathon swim from Tiran Island in Saudi Arabia to Hurghada in Egypt posed a myriad of difficulties, not least because it involved weaving through shipping traffic in the Gulf of Suez -- the stretch of water connecting the Suez Canal to the Red Sea.
And if negotiating a steady stream of oil tankers and freight containers wasn't problematic enough, Pugh was also buffeted by big, rolling waves as he fought against choppy waters for the majority of the swim.
In total, he covered a distance of roughly 76 miles (123 kilometers) from October 11-26, swimming between 3.5 and 7.5 miles each day.
"My body is really, really hammered," says Pugh, about a week after finishing the swim. "Every single day, these waves were crashing up against me ... It was just twisting my body backwards and forwards, backwards and forwards."
The swim also carried the ever-present risk of encountering a shark, of which there around 40 different species in the Red Sea, according to Pugh; the most dangerous are hammerheads, oceanic whitetips, oceanic blacktips and tiger sharks.
As protection, the underside of Pugh's support boat was equipped with an electronic device capable of repelling sharks within a four-meter radius, meaning any run-ins were few and far between.
But the sea life Pugh did witness at close quarters left him mesmerized by its beauty.
"When you swim across these coral reefs, it's absolutely unbelievable because the colors are so vibrant -- the yellows, the purples, the greens, and then all the wildlife that lives in them," he says.
For sections of the swim, Pugh was joined by open-water swimmer Mariam Saleh Bin Laden -- who became the first Arab, first Saudi and first woman to swim from Saudi Arabia to Egypt -- and Egyptian swimmer Mostafa Zaki.
The purpose of the swim was to shine a spotlight on the world's coral reefs -- home to the earth's most vibrant marine ecosystems -- and their precarious status amid the climate crisis.
Scientists have predicted that about 70% to 90% of all living coral will disappear in the next 20 years in the face of rising sea temperatures.
According to findings from an Australian government agency published earlier this year, warming waters have already caused coral bleaching in 91% of reefs surveyed along the Great Barrier Reef.
Pugh, a leading figure in marine protection as the UN Patron of the Oceans, says the coral and wildlife in the Red Sea have adapted to the high water temperatures over thousands of years, making it home to some of the most resistant coral in the world.
But other places tell a different story.
"I did a swim a few years ago across the width of the Maldives -- a group of islands in the middle of the Indian Ocean -- and I remember just swimming over these coral reefs, and they were absolutely unbelievable," says Pugh.
"I went back 10 years later. The water had risen; the temperature of the water had risen just a little bit; the animals had all but disappeared, and that coral was completely white, bleached, dead."
This week, Pugh has traveled to the COP27 climate summit in Sharm El-Sheikh, Egypt -- a location he passed during his swim across the Red Sea.
There, he plans to speak with world leaders about the gravity of the climate crisis and what it means for the future of the planet -- just as he did at last year's COP26 in Glasgow, Scotland, after swimming across Greenland's Ilulissat Icefjord.
"I see the polar regions and the coral reefs of the world as the two ground zeros of the climate crisis," says Pugh. "And the reason why I say this is because it's so evident in these parts of the world that we have a very, very serious crisis."
Part of the reason for Pugh's long-distance swims is to persuade world leaders to introduce marine protected areas.
In 2015, for example, he swam down the Ross Sea in Antarctica, which today contains a protected area spanning 1.55 million square kilometers -- the largest such area in the world at roughly the size of the United Kingdom, Germany, France and Italy combined.
But Pugh also wants the swims to tell stories about parts of the world which are often overlooked.
"When you see damage on land, it's so very, very evident," he says. "Underwater, it's much more challenging. With these swims, I tried to take people -- take the public, take the media, take world leaders -- to the scene of the crime and show them what's happening and explain why it's important that we protect these places."
Still recovering from his Red Sea swim, Pugh is unsure of the next ocean he will plunge into wearing just his swimming briefs. For now, he is focused on COP27 and the promises made by the world's leaders in the face of the climate crisis.
"We need to have commitments which are much shorter, much sharper," says Pugh. "And our commitments need to be far greater than what I've seen before."