Lots of UK beaches have sewer overflows close to them which direct raw sewage into the sea after big storms.
The Marine Conservation Society (MSC) is advising people to wait a few days before going for a swim after rain.
They're also calling for signs on beaches to warn swimmers about potential health and sewage hazards.
Wait to swim
"We would ask swimmers to avoid going in the sea for a day or two after heavy rain," said Thomas Bell from the MCS.
The advice follows weeks of rain in August, which lead to the Cornish village of Boscastle being flooded and a big landslide in Scotland.
Storms in London earlier this month sent 50,000 tonnes of raw sewage into the River Thames, when sewers were flooded.
Thousands of fish also suffocated in the polluted water.