on this link, the institudo estadual do ambiente - inea, provides the suitability of several beaches on ilha do governador, including praia das pelônias / bancários, which is on the same side of the island as praia do saco pinhão, and can be loosely 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.