Up to 150 people reported feeling ill after visiting beaches on the Essex coast over the bank holiday weekend.
Symptoms affecting beachgoers in Frinton-on-Sea, Clacton and Walton-on-the-Naze included breathing difficulties, vomiting and coughing.
Fourteen people were taken to Colchester Hospital after being in the water but no-one was seriously ill.
A spokesman for Tendring District Council said the source of the illness was still unknown.
People first reported feeling unwell at about 14:00 BST on Sunday.
A woman described how her twin daughters were 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."
There have been no additional reports of people becoming ill since the initial outbreak of symptoms.
Water samples were taken for testing but the results are still pending.
Beachgoers are being advised to stay out of the water until a cause has been established.
The Maritime and Coastguard Agency said there was no evidence of a fuel spillage from vessels.
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There have been previous cases of people becoming ill on beaches during the summer.
- On 11 August, Worthing seafront was evacuated. Investigators concluded hazardous material could have come from a nearby boat.
- In August 2017, about 150 people attended hospital in Eastbourne reporting sore eyes and vomiting. An investigation suggested a gas cloud came from the Channel.