on this link, the institudo estadual do ambiente - inea, provides the bathing suitability of several beaches in the sepetiba bay, including praia da brisa, that can be used to guess water conditions at praia da pedra de guaratiba. 

constantly updated, the red flags mean that swimming is not advisable, green flags mean waters are clean, and yellow flags suggest using own judgment.

unfortunately, while cleaning-up efforts are underway, many of the beaches in the sepetiba bay are normally not often suitable for swimming.