RIO DE JANEIRO — Health experts in Brazil have a word of advice for the Olympic marathon swimmers, sailors and windsurfers competing in Rio de Janeiro’s picture-postcard waters next month: Keep your mouth closed.

Despite the government’s promises seven years ago to stem the waste that fouls Rio’s expansive Guanabara Bay and the city’s fabled ocean beaches, officials acknowledge that their efforts to treat raw sewage and scoop up household garbage have fallen far short.

In fact, environmentalists and scientists say Rio’s waters are much more contaminated than previously thought.

Recent tests by government and independent scientists revealed a veritable petri dish of pathogens in many of the city’s waters, from rotaviruses that can cause diarrhea and vomiting to drug-resistant “superbacteria” that can be fatal to people with weakened immune systems.