on this link, the institudo estadual do ambiente - inea, provides the suitability of several beaches in the west zone, including praia de barra de guaratiba (praia grande) which is next to praia do canto and can be used to guess water quality there.

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.