I'm so going to get buried by mods who disagree rather than marked informative, but here goes.
I moved to Rio 6 months ago from New Jersey. (Yeah, I know, NJ is the land where every puddle has its own rainbow.) I know 4 other people who moved here from the US within the last 2 years. Not one of us has gotten sick.
How often do you swim in the rivers/lakes there, or do other water sports?
Okay, I'll stop with the anecdotal evidence now. How about, this place is CLEANER than New York City. How many folks swim in the Hudson or the East River?.
Nobody is trying to have an Olympic event in the Hudson or the East River.
Stop playing telephone and ask someone who's been there - Rio is Fine; far cleaner than many other cities I've seen.
Maybe so, but that doesn't have anything to do with the fact that the lakes/rivers are so polluted that you shouldn't be having athletic competitions in them.