One of the participants, Australian triathlete Jake Birtwhistle, said on Instagram that he had been "feeling pretty rubbish since the race."
"The swim should have been canceled," he added, posting a picture of a graph with the results of the water tests from Roker.
The competition was held in an area of the coastline that has been at the heart of a long-running dispute between campaigners and the government over sewage discharges, the Guardian newspaper reported.
Local water company Northumbrian Water insisted there have been no discharges "that might negatively impact water quality" at Roker since 2021.
The Environment Agency said "temporary dips" in water quality can be caused by various factors "including heavy rain".