due to safety concerns, we recommend going to ilha do governador's beaches via taxi, and / or only accompanied by a professional guide or someone familiar with the area - a taxi ride costs about br$ 70 from rio's zona sul, each way. on this link, the institudo estadual do ambiente (inea) provides the suitability of several beaches on ilha do governador, including praia da engenhoca, which is very close to praia do zumbi, 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.