Taking a dip in Copacabana and Ipanema
Ipanema had a lot of problems with pollutions 10 years back, until they moved the exit of the sewer.
Being on one of the most famous beaches in the world, sweating in the sun and having lots of hot women in the waves... of course I enter. It's mostly for a dip or some bodysurfing. I can't imagine it would be that bad as the descriptions below.
The only problem I had was once when I accidently happened to catch some human faeces in my hand. :(
Copacabana and Leblond beaches
If you go up to Sugar Loaf and look down to the ocean, the water at Copacabana beach looks greenish blue, while the water in the bay next to Leblond and Centro looks black like mud.
It's obvious that Rio's sewage and municipal wastes are pumped out into that bay. The contaminations would gradually spread out into the surrounding water. The ocean water in Copacabana does smell like sewage. It's not smart to dunk around in water that carries human wastes of million of people for decades.
The water over Ipanema looks cleaner but the beaches are littered with trash of every kinds. The water in resort town, Buzzio, is also heavily contaminated with sewage.
There are many beaches in other parts of the world that are beautiful, clean, healthy, fun. Rio is very low on the list. I was told by my Brazilian friends that Southern parts of Brazil have far cleaner beaches and more beautiful women of the blonde hair, fair skin types.