People in Jersey are being urged not to swim in the sea or walk in any standing water as it could contain raw sewage.
The warning comes after more than half a month's rain fell in just 19 hours across the island.
Jersey's Chief Minister, Kristina Moore, issued the warning in an emergency press conference on Tuesday alongside the heads of the emergency services.
It comes after a major incident was declared and 39 families were evacuated from the Grands Vaux area as the floodwater rose.
The head of Jersey Water Helier Smith said: "The advice is very clear, you should avoid allowing your children or pets to play in standing water until the advice comes out from Public Health.
"There are risks so avoiding playing in puddles until they're cleared away would be recommended."
Jersey's Director of Public Health, Peter Bradley, has also warned against sea swimming for the next 48 hours.
Signage will be put up to warn people about the risks and possibility of sewage in potential swimming water.
Jersey's Chief Fire Officer Paul Brown added that people should not be entering floodwater anyway as drain covers may have moved and there could be hidden dangers.
Mr Smith went on to reassure residents that the mains water supply is not affected, although those with boreholes should take some precautions.
He explained: "In respect of people on boreholes, I would advise some caution, especially if you live near a sewage pumping station because they have been overtopping.
"I would personally recommend that you boil water before consuming it and I think Public Health is bringing out advice specifically on that."
It was one of the biggest news stories of our time - and it's still not over. So what did Boris Johnson know about Downing Street’s notorious parties?
With fresh revelations from our Number 10 sources, in their own words, listen to the inside story...