Endurance swimmer Lewis Pugh has swum the full length of the Hudson River to raise awareness of water quality.
Mr Pugh arrived at Battery Park, Manhattan, on Wednesday - a month after starting the swim at Lake Tear, in New York State, on 13 August.
The 53-year-old activist from Plymouth, Devon, covered 315 miles (507km).
"Rivers are the arteries to our planet and if we are going to have a healthy planet we need to have healthy rivers," he said.
"All I could think as I was swimming towards the Statue of Liberty is every single thing we hold dear to ourselves relies on us being able to drink clean water and breathe fresh air and take care of our planet so it's habitable," he added.
Mr Pugh timed his arrival in Manhattan, home of the United Nations Headquarters, to coincide with the countdown to the 2023 UN General Assembly and the United Nations' ratification of the High Seas Treaty.
The treaty aims to protect biodiversity in international waters.
Mr Pugh said his goal was to present a letter to the UN, signed by thousands of people across the world.
It calls on world leaders to honour their agreement made at the Convention on Biological Diversity in 2022 which pledges to protect 30% of the world's land, inland waters and oceans by 2030.
It includes a promise to prioritise the restoration and revival of vital river systems.
Mr Pugh said he swam for the cause of "clean, fresh water and rivers", adding: "It was an incredible challenge and today I'm feeling absolutely exhausted but I'm also feeling incredibly motivated."
He said historically the Hudson was "so badly misused" that it was one of the world's most polluted rivers but that a "mile by mile" mission to clean it up had yielded "amazing" results.
He said his swim would have been impossible 50 years ago as he would have "just got too sick".
Mr Pugh is known for swimming across the North Pole in 2007 with the aim of highlighting the melting of the Arctic sea ice.
In 2018 he swam the full length of the English Channel.