Five swimmers have been rescued but one person remains missing after getting into 'difficulty' at Clacton pier.
Essex Police have confirmed they are dealing with an "incident" with "significant emergency services" present.
Lifeboat crews have been deployed as the search for a man missing in water continues as an air ambulance joins the rescue attempt.
Of the five people saved so far, one male and one female have to be taken to Colchester Hospital for further treatment.
The Police said: "We're on the scene assisting emergency services colleagues with a serious and ongoing incident close to Clacton Pier.
"There is a significant emergency services presence in the area while the incident is being dealt with. We'll provide an update as soon as we practically can."
Clacton Pier has closed and members of the public enjoying the hot weather have been asked to leave.
East of England Ambulance Service issue statement confirmed one male is still missing.
A spokesman said: "A large response, including two ambulances, four Hazardous Area Response Team vehicles and Essex and Herts Air Ambulance, was sent to Clacton beach this morning following reports of a number of people in difficulty in the water.
"Five patients were assessed at the scene. One male and one female were transported to Colchester Hospital for further assessment and care. Three males were discharged at the scene.
"Crews remain on-site as one male remains unaccounted for."
Britain is enduring the hottest day on record as temperatures reached 40C just before 1pm today - and it's set to get even hotter.
An HM Coastguard spokesman said: "HM Coastguard is currently responding to an incident at Clacton Pier following a report of multiple people in the water.
"Clacton Beach Patrol, the RNLI lifeboat from Clacton and coastguard rescue teams from Clacton, Walton and Holbrook have been sent to help, as well as the coastguard rescue helicopter from Lydd. Essex Police and Essex Ambulance service are also attending along with Essex Fire and Rescue Service.
"Five people have been rescued, with the search still ongoing for a sixth man."
Witness Nigel Brown told the BBC: "People got into difficulty in the water just around the pier. They seem to have got caught it in the tides and the currents and drifted off the beach towards the pier.
"Three people have been rescued and one is still being searched for."
Earlier, a man in his 20s died at the Cotswold Water Park, Wiltshire Police said.
Police were called shortly after 6pm on Monday and said the man was confirmed dead shortly after being pulled from the water.
His death is not being treated as suspicious.
The park is an area of more than 40 square miles with 180 lakes.
