Emergency services have been called to the seafront in Essex after receiving several reports of people, including children coughing and struggling to breathe.
Beach-goers who had been visiting the popular seaside town of Frinton on Sea isaid they had been left "gasping" for air after swimming in the ocean.
Police, the ambulance service and the fire service were called to the seafront off Fourth Avenue in Frinton, after receiving calls shortly after 2pm on Sunday.
A mother, who was on a family day out at the busy beach, told how her one of her twin daughters was left "gasping" for breath.
Miriam Lansdell said: "My daughter started coughing. She said 'I don't feel good. It hurts to breathe in'.
"My other daughter was gasping and couldn't form words because she couldn't breathe well enough."
The 45-year-old mental health worker from Derbyshire, who was visiting her parents in Essex for the bank holiday weekend, said she had also had difficulty breathing as she lay on the sand drying off after a dip in the water.
She said they all began to breathe easier when they moved further away from the beach, but took the 10-year-old girls to a walk-in clinic to be checked over by medical staff.
Speculation online of a fuel spill was not confirmed is being investigated.
Ms Lansdell said her father had been told by someone in a speedboat, who he assumed to be associated with the coastguard, that there may have been a fuel spill.
She said: "My dad said he had been asked to get out of the water by a man on a boat. He asked why and the man said there had been a fuel spill. He said if anyone is having breathing difficulties they should probably call an ambulance."
She added: "It's not what you expect when you go for a day out to the beach."
One person tweeted that there were "lots of people coughing heavily", while a mother said her son began coughing after swimming and had to be given his inhaler.
A spokeswoman for East of England Ambulance Service said: "We are aware of an incident on Sunday 25 August with reports of a number of people suffering from coughing on the seafront off Fourth Avenue, Frinton.
"We are assisting the police and fire services with this incident. The cause is currently unknown."
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