Before he got in the water on Saturday, Lewis Pugh was terrified.

Mr. Pugh, 47, is a British endurance swimmer who has conquered a gantlet of extraordinary swims in the last 30 years, from a dip in a glacial lake in Mt. Everest to a full circuit around the Cape of Good Hope in South Africa, where he was raised. He has no problem with swimming, per se.

But on Saturday, he was set to dive into an ocean that was 31 degrees Fahrenheit (-0.5 degrees Celsius), situated along the edge of an Arctic ice pack. Had it been fresh water, it would have already been frozen.

“You just don’t know how your body’s going to respond to this type of thing because no human’s ever done it before,” Mr. Pugh said in an interview on Tuesday. “All the fear is on your shoulders.”

The swim, which took place north of the Norwegian island of Spitsbergen, constituted the first leg of a tour Mr. Pugh is taking of the Arctic Ocean as part of his role as the United Nations patron of the oceans, in which he advocates on behalf of ocean health.