on this link, the institudo estadual do ambiente - inea, provides the suitability of several beaches on ilha do governador, including praia dos bancários / das pelônias, which is close to praia dos bancários and can be used to guess water quality.  

constantly updated, a red flag means that swimming is not advisable, a green flag means waters are clean, and a yellow flag suggests using own judgment.

any of rio's beaches may have polluted waters within 24 to 48 hours following heavy rains - while cleaning is underway, many of ilha do governador's beaches are normally not suitable for swimming.