on this link, the instituto estadual do ambiente (inea) provides the suitability of several beaches, including prainha and pontal de sernambetiba, that can be used to determine water conditions at macumba beach.   

constantly updated, the red flags mean that swimming is not advisable, green flags mean waters are clean, and yellow flags suggest using own judgment - any of rio's beaches may have polluted waters within 24 to 48 hours following heavy rains.      

this link shows praia da macumba on a wikimapia - the link as we present it is best seen using google chrome, as zoom settings it might not open correctly with other browsers. on this link an early morning shot of the beach is posted daily, showing water conditions.