The quality of the sea water in Cannes is largely excellent. Samples are taken weekly from mid-June to mid-September and analyzed for fecal and bacteriological contamination. The results are regularly posted at the entrance to beaches as well as on the Cannes municipal website. For a yearly overview of sea water quality in Cannes and elsewhere in France, see the Ministry of Health.
A review of the reports issued by both sites reveals that most of Cannes's sea water is completely clean all the time. The exceptions would be the beaches from Porto Canto on east to Mouré Rouge which occasionally have slightly elevated pollution levels, presumably from boats clustered around the Porto Canto port.