It wasn't so much the microbes but the wind that made waves during Olympic rowing.
For the first time since 1996, the Olympic regatta was not held in a purpose-built lake, but a natural lagoon that left rowers exposed to the elements in new ways.
While worries about water pollution marked the run-up to the Rio de Janeiro Games, it was the wind swishing in from the mountains that stirred up trouble for rowers in narrow, unstable racing shells.
American sculler Genevra Stone felt right at home in the choppy waters, which reminded her of her hometown.
"This is classic Boston basin," she said after winning silver in the women's single sculls. "Same wind. We got some wake bounce and I was just like, 'This is my thing. I can row through wakes.'"
The British team also seemed comfortable in the conditions and topped the rowing medals table with three golds and two silvers.
Others had a tough time.
"I was pretty close to sinking out there, which generally would be an indication that the course isn't rowable," Australia's Kim Brennan, the world's top female sculler, said after the first day of the regatta.
She looked hopelessly lost amid the whitecaps on the lagoon, finishing her opening heat in third place behind Kenia Alanis of Mexico and Micheen Thornycroft of Zimbabwe, rowers who normally wouldn't be able to match her pace.
"It was chaotic," Thornycroft said. "It was anyone's game. Whoever could get their blades in and move."
Brennan learned from the experience and dealt with the conditions better in subsequent heats. She led the final Saturday from start to finish to win Australia's first Olympic gold in women's single sculls.
"This week was a reminder that a top rower needs to be truly ready for all conditions," said Matt Smith, executive director of the international rowing federation.
At least two boats capsized during the regatta, which is unusual in elite races. Serbia's men's pair overturned in its opening race and Kazakh sculler Vladislav Yakovlev flipped in two consecutive heats. The episodes received more attention given the concerns about water pollution in Rio de Janeiro.
Hamish Bond, gold medalist with Eric Murray in the men's pair, likened the conditions on the first day of racing to "walking down some stairs and when you're mid-stride the step gets three times the height."
But he and Murray knew what to expect in Rio and said they had prepared for the conditions.
The Rodrigo de Freitas Lagoon is separated from the Atlantic Ocean by a strip of land that holds the Ipanema and Leblon neighborhoods. The lagoon is much bigger and doesn't have the rectangular shape of an artificial race course.
On two days, the winds were so strong that races were called off, but even on calmer days the water could be choppy.
Conditions are expected to be easier for kayakers and canoeists racing on a shorter course for sprint events next week.