Argentina's Damian Blaum claimed a home victory as the International Swimming Federation (FINA) Open Water Grand Prix season began in Santa Fe.
The 35-year-old won the men's race after touching home in 8 hours 28min 16sec, following an arduous 57 kilometres course.
He was more than five minutes in front of second-placed Italian Edoardo Stochino, who finished in 8:33.50.
Evgenij Pop Acev of Macedonia did enough for bronze, ending the race only a fraction behind Stochino.
In the women's race, Italy claimed the top two positions on the podium.
Barbara Pozzombo was triumphant as the only swimmer to come home under the nine hour mark, finishing in 8:53.42.
Her team-mate Alice Franco was second but well adrift on 9:01.20, with Aurelie Muller of France third in 9:05.13.
The season will continue in Lac St Jean in Canada on July 29.
Ohrid Lake in Macedonia will host the third leg on August 19, before the Grand Prix circuit concludes in Capri-Napoli in Italy on September 3.