Sailors have expressed disgust at the filthy state of the Brazilian waters in which they will race at the 2016 Olympics, with a former British star describing it as a "sewer".
They spoke out after visiting the Rio de Janeiro venue yesterday.
He said sailors in training ad to stop to disentangle their rudders from rubbish.
"It's a sewer," he said. "It's absolutely disgusting. Something has to be done about it. But you need the political will for these things to happen and at the moment it's not there."
"I've been sailing all over the world for 20 years now, and this is the most polluted place I've ever been," said Allan Norregaard, a Danish bronze medalist in the 2012 London Olympics.
"It's really a shame because it's a beautiful area and city, but the water is so polluted, so dirty and full of garbage."
Rio's Olympic organising committee has promised the pollution will be cleaned up when the Olympics open and the government has pledged to reduce 80% of the pollution flowing into the bay.
But the sailors doubt the problem can be fixed after festering for decades, and many worry about their health. Environmentalists say measures being taken are a "stopgap" likely to mask the problem, not cure it.
Over the last several weeks nearly 70% of Rio's waste goes untreated into surrounding waters, according to figures compiled by The Associated Press news agency.
Famous beaches like Copacabana and Ipanema are dirty and untreated sewage pours into a lagoon bordering the Olympic Park, the heart of the games.
Norregaard said that while sailing the last few days he saw entire trees floating in the bay, doors, chunks of timber with nails protruding, swollen mattresses and endless plastic bags.
Another sailor talked about a horse carcass in the 148-square-mile bay, which opens into the Atlantic just above Rio's famed Copacabana beach.
The Dane said the floating debris made racing unfair and dangerous. The other issue is the health risk with high levels of fecal coliform bacteria in the water.
"I would definitely not swim in it," Norregaard said. "We have had a couple of incidents where people went in the water and came up with red dots on their body. I don't know what's in the water, but it's definitely not healthy."
Brazilian sailor Martine Soffiatti Grael grew up on the bay. Her father, Torben Grael, is a five-time Olympic medalist, two of them gold.
"For me since I was a child, it has only got worse," said the 22 year old, who hopes to qualify for the Rio games. "The government says it has lots of programmes to clean the bay, but I haven't seen any progress being made."
Thomas Bach, the new president of the International Olympic Committee, is due in Rio early next year to monitor progress. The IOC is concerned about delays in organising and building venues, and pollution is another worry with costs for the games put at £9.2 billion - a mix of public and private money.
"Of course, the water will not be clean as sailing in the Caribbean," Brazilian Robert Scheidt, who has won five Olympic medals, said. "I have never swum in there (Guanabara). ... Inside the bay I know it's not the proper place to swim. I've sailed there and never got any disease."