RIO DE JANEIRO - Olympic sailors gave Rio's polluted waters a thumbs-up as competition got underway -- although some warned floating garbage remains a hazard at the "filthy" venue.

At least one windsurfer caught a plastic bag in the opening races on Monday, although there were no serious incidents like the capsize of a catamaran that hit debris in training. Despite worries over the dangers of floating trash, and bacteria from raw sewage, most sailors questioned by AFP were upbeat.

"This is like perfect conditions. You can't get better than this," said the United States' Pedro Pascal, who windsurfed in one of the opening events on Monday. It's beautiful, as you can see," he said, looking back over the beach at Sugarloaf Mountain and Guanabara Bay.

Sailors said the way the wind swirls around the dramatic landscape and cities enclosing the bay makes tactical decisions hugely challenging. And they like that. "I think it's the best place to sail in the world in a technical sense," said Denmark's Anette Viborg, who is competing in the Nacra 17 catamaran with Allan Norregaard. It's very difficult. The wind changes a lot because of the mountains," she said, after pulling the boat out from a training session.

US Finn class sailor Caleb Paine, who was also out training Monday, called the water "very nice". If floating garbage is a risk for slowing down boats, then so is seaweed, he said. "Every place has a little twist to it."  About half of the sewage from greater Rio pours untreated into Guanabara. So do mountains of trash dumped into rivers that feed the bay. To stop the garbage from ruining races the authorities have placed nets across the rivers and deployed a fleet of garbage-collecting boats called "eco-barcos" to try and scoop up whatever escapes. One of the green boats could be seen patrolling near the races Monday.

But despite the clean-up measures, the waters are far from pristine. "It's filthy," declared Tunisia's Hedi Gharbi, who races a Nacra 17 with crewmate Rihab Hammami -- the first catamaran Olympians ever from their country.

 "We hit something the other day and capsized when we lost control," he said, although he praised Rio's organization in general.

Italian windsurfer Mattia Camboni said after taking part in the first races Monday that the water quality has improved over the months he has been coming to train in Rio. But the danger's still there. "I saw the French guy just in front of me in the last race and he caught something," Camboni said.

A small plastic bag will marginally slow down a windsurfer. So "if the plastic bag is not very big then you jump, using the power of the sail, and hopefully it goes away," Camboni said. Large bags are different. They can ruin a race. "You have to stop and put the sail in the water" to get down and pull it off, the Italian said.

Ioannis Mitakis, a Greek Finn class sailor, was philosophical about the hidden dangers out there. "About the rubbish in the water, it's the same for everybody, so you have to take care of that," he said. "My old ex-coach says that luck is with the good ones. Sailing is a lucky sport but the best always win."