Tourism chiefs have asked beach goers to avoid the water after a number of people were struck by sudden and unexplained ill-health.
Emergency services were called to the seafront after reports of people coughing and struggling to breathe on Sunday.
The council said there had been no further reports of illness since a number of people reported feeling unwell at beaches in Walton, Frinton and Clacton yesterday - with symptoms including irritation to the eyes and breathing difficulties.
Some were taken to hospital for further evaluation, said Tendring District Council, which would not speculate on the cause but said there was no reason for alarm.
Beach goers visiting Frinton-on-Sea said they were left "gasping" for air after swimming in the sea.
All three primary emergency services were called to the beach off Fourth Avenue just after 2pm.
Alex Porter, district head of leisure and tourism, said no one was seriously hurt, and that it would be unwise to speculate on a cause until the completion of investigation work.
Beach goers were encouraged to respect advice and avoid the water, as beach patrols were stepped up on Monday.
Louise Harvey visited Frinton beach from Needham Market with husband Nick, 11-year-old daughter Poppy and 13-year-old son Harrison on Sunday.
Within about 40 minutes of going for a swim, both children began coughing and wheezing.
Mrs Harvey, meanwhile, who had been paddling with her husband, soon felt a burning sensation in her nose.
After picking up fresh water nearby, the family returned to meet paramedics and allow the children to be checked over.
"After a couple of hours, the children's coughs had almost gone," said Mrs Harvey.
"But the crackly feeling in their chests was still there - and we all still had sore throats and noses."
Mrs Harvey said the water was clear, but, in hindsight, recalled smelling fumes and seeing a haze in the air. She praised the speed and professionalism of lifeguards, response teams and emergency services.
The district council said visitors could still enjoy the beaches, promenades, seafront gardens and parks without venturing into the water on Monday.
Mr Porter added: "Coupled with numerous businesses selling cold drinks and ice creams to help visitors beat the heat, Clacton, Frinton and Walton remain good places to visit for bank holiday Monday."
Anyone coming into contact with the water and feeling unwell was advised to rinse and change clothes, drink fresh water, and call the non-emergency health number on 111 in the event of further concerns.
Will Lodge, borough council communications manager was hopeful that, with two tides passing through, the cause of this incident will have also passed.
"However, we will not be complacent and will continue to review the situation," he added.
The sudden outbreak of illness came almost exactly two years after beaches in East Sussex were evacuated following a suspected chemical leak.
More than 200 people were treated at Eastbourne General Hospital for the effects of a chemical haze - most likely caused by a ship, a wreck or lost cargo in the Channel.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here