Some participants in Saturday's event complained about the state of the water in Copacabana, which was distinctly brown and dotted with plastic bags and other detritus.
"Last year, when I got out, a diaper was stuck in my suit, and this year there were lots of plastic bags everywhere," said Luan Kawata, an 18-year-old high school student who was among the top finishers in the 1-kilometer-long sprinting event. "The water's quite dirty and that's a problem."
Roberto Pereira Rodrigues, a 55-year-old triathlete, complained that recent rains, which regularly flush raw sewage and trash into the ocean, muddied the waters for Saturday's event.
"It happens every time it rains," he said with a sigh as he applied Vaseline along the neckline of his wetsuit.
Activists from nongovernmental group Meu Rio unfurled a banner reading "We don't want to swim in sewage anymore!" It's pushing for large-scale infrastructure projects to bring basic sanitation to much of the city,
Rio's state government has pledged the city's waterways will be clean by the Olympics and insisted in a statement earlier this week that "the health and welfare of the athletes is always our top priority."
The statement also detailed efforts that are underway to clean up Rio's waters, including barriers at the mouths of rivers that trap floating debris and a fleet of garbage boats that ply the waters, removing larger pieces of detritus that experts warn could prove dangerous for competitors in the Olympic sailing events.
The United Nations' special rapporteur for water and sanitation issues, Catarina de Albuquerque, said that while progress has been made, the situation here remains dire.
"What I saw in Rio was that people who are on the margins of society, people who are poor . still do not enjoy the human right to water and sanitation. That's the reality," Albuquerque said.
Luiz Lima, a Brazilian swimmer who competed in the Atlanta and Sydney Olympics and won the King of the Sea competition in 2008, compared Rio's waters to Sydney Harbor.
"It was really polluted there, but they cleaned it up remarkably ahead of the Games," said the 37-year-old. "I just hope our waters will be like Sydney Harbor by the time the Olympics are over."
- AP