I was surprised to find no mention of this on the forums here, was reading about the PFAS issue affecting other Australian airports and then looked into Botany Bay.

Seems that there is certainly contamination in the water (they advise against eating too much fish), but no advice on entering the water. The readings from Botany have not been released.

www.smh.com.au/national/secret-documents-reveal-toxic-threat-beneath-sydney-airport-20181220-p50nfn.html

www.epa.nsw.gov.au/your-environment/contaminated-land/pfas-investigation-program/pfas-investigation-sites/botany-bay-area

www.theleader.com.au/story/5472712/botany-bay-and-kurnell-toxins-probe/

Thoughts?! Seems slightly concerning to me...