Yachts and fishing boats converged Saturday on Rio's filthy Guanabara Bay, the site of next year's Olympic sailing contests, to protest the authorities' failure to tackle severe pollution.

More than a dozen sailboats, eight small fishing craft and a swarm of canoes were among the fleet embarking from Marina da Gloria, the inner city harbor where Olympic athletes are already preparing for the 2016 Summer Games.

Sailing under the imposing cliffs of the Sugarloaf mountain, protesters honked foghorns, blew rescue whistles and chanted "baia viva!", meaning roughly "the bay lives!"

Water quality in Guanabara has become the main concern over Brazil's preparations for the world's biggest sporting event. The city promises safe conditions, but environmental activists say the stunningly scenic Guanabara Bay is effectively a giant sewer.

"There's not a single beach in the bay that is unpolluted," said environmentalist Sergio Ricardo, who joined the protesters.

Not only does a majority of the greater Rio area's sewage pour untreated into Guanabara, but dozens of rivers deliver a steady supply of trash, environmentalists say.

According to Ricardo, in some areas of Guanabara "there's a more than one-meter thick layer of plastic at the bottom."

Rio's original bid to host the Olympics included the headline promise of cutting pollution by 80 percent, but officials now concede this has no chance of being achieved any time soon.

"It was propaganda," Ricardo said, "a completely unrealistic goal."

Also taking part in the floating protest, Brazilian Olympic sailor Isabel Swan spoke of her love for Guanabara Bay.

"It's the most beautiful postcard in the world and it lacks just one thing -- being clean," Swan, who won bronze in Beijing in 2008, said.

- Battle for public opinion -

Environmentalists see the run-up to the Olympics as a unique -- possibly final -- opportunity to pressure the government into making good on a clean-up program that has been running with little effect for two decades.

Saturday's protest would not have been immediately visible to many Rio residents, unless they were among those sunbathing on one of the nearby polluted beaches. However, a boat carrying a large contingent of Brazilian TV crews and other journalists alongside the demonstrators ensured an impact.

"That's the idea of this," said one of the organizers with the Baia Viva group, Nahyda Franca, 58. "We're one year from the Olympics and need to get attention."

The pollution is impossible to deny. Even inside Marina da Gloria on Saturday raw sewage and toilet paper could be seen pouring from city drains.

However, officials and environmentalists don't always agree on the scope of the problem or the solution.

This week, Rio Mayor Eduardo Paes said athletes would be "protected," while International Olympic Commission boss Thomas Bach said conditions would be "good."

Ricardo said the real story is told by the bay's much-loved dolphin population, which he said had gone down from "hundreds" a couple decades ago to 35 -- two down in just the last three months.

"This is a failure," he said.

Sailing team members preparing in the marina for an Olympic dress rehearsal next week appear to have set aside any worries.

Guillaume Chiellino, manager for the French team, said his sailors are taking medical precautions and that independent tests of the water will be made.

However, he was "delighted" to be in such a "mythic bay."

Olympic sailors are often stranded far from the buzz of the host city, as was the case in the 2012 London games. Being in Guanabara Bay this time means a chance to take center stage.

"For our sport it will be grand to have the images under the Sugarloaf mountain and the Corcovado," Chiellino told AFP at a storage area for sleek racing dinghies. "I think we can be the sport of these Olympics."

"That, I think, will help us forget that the water isn't transparent."