World Sailing says the water quality at the Rio 2016 sailing venue needs major improvement, as organisers prepare to open a new pipe belt to divert sewage.
Tests at Guanabara Bay have revealed high levels of bacteria and viruses coming from human sewage.
Two sailors contracted infections at a test event in August, which they claim were caused by the waters.
Rio Olympic organisers said earlier this month that the health and safety of athletes is "always a top priority".
Seven of the 10 sailing events in the Brazilian city will launch into the Marina de Gloria, where the new pipeline is due to open this month.
"The Marina area is our biggest challenge; it's critical it's completed and we start seeing a difference," World Sailing's head of events Alastair Fox told BBC Sport.
"It's essential that it improves radically - from a health point of view and also for showcasing the sport."
Nearly 70% of sewage in Rio is spilled raw into its surrounding waters.
Sailors have also reported seeing pollution, including furniture and floating animal carcasses, while drug-resistant "super-bacteria" were found in the area last December.
A statement from the Rio 2016 organising committee earlier this month said: "There is no doubt water within the field of play meets the relevant standards."
The Games begin on 5 August.
'A gold medal makes up for anything'
However, World Sailing says the 8% illness rate at the event was lower than average for regattas.
It also says doctors feel those illnesses could have been prevented, and believes athletes can minimise the risks by following hygiene guidelines, such as taking showers after exiting the water.
British two-time Olympic champion Sarah Gosling, now the athletes' representative on the World Sailing council, believes the pursuit of Olympic gold will outweigh any risks involved in the minds of competitors.
"Winning a gold medal will make up for anything - it's really not that relevant," Gosling told the Associated Press on Tuesday.
"There are plenty and plenty of reports out there about athletes who their whole life is about winning medals, and anything that happens on the way is kind of irrelevant.
"For sailing to be in the centre of the Olympics right there, for athletes to be able to stay in the village, to be able to go to the opening and closing ceremony, it's a massive deal."
'Sailors want to race in Rio'
World Sailing has had reports the rubbish in the water at Guanabara Bay, where Marina de Gloria opens into, is higher than usual because of rainy season deluges washing items into it.
Fox will conduct the latest site visit in late January to assess progress.
"We're doing as much as we can to work with the Rio authorities," he said, adding World Sailing also had back-up plans to treat the water should the work not be completed.
"When racing near a big city, water quality is always an issue. We've no intention of not holding the competition there. The reality is that sailors want to be racing in Rio.
"We need to start seeing data from all the course areas in the dry season to see that water quality is at an acceptable level - as they were in August at the test event in the Guanabara Bay."