Two teenagers have been taken to Colchester Hospital in a critical condition after being pulled from the water in Clacton.
One witness, who asked to remain anonymous, said: "Three of them were pulled out of the water and two of them looked really bad.
"There was members of the public out there giving them mouth to mouth before the beach patrol got to them."
An ice cream vendor on the seafront added: "It's another tragedy on our beach. It happens every year, there's swimming spots at each end of the beach but these kids were swimming in the middle.
"People come from all over and hey don't know what the waters are like."
A spokesman for Essex Police confirmed that those pulled from the sea were teenagers.
They said: "Emergency services were called 1.40pm today to reports that three people were recovered from the water near to Clacton Pier and were being treated on the beach.
"We are all in attendance and the three casualties are still being treated.
"Two remain in a critical condition."
A number of air and land ambulances were called to the scene.
A spokesman for the East of England Ambulance service said: "We were called to an incident today (8th August) at 1.34pm at Marine Parade, Clacton on Sea.
"Our first response vehicle arrived at the scene in under seven minutes.
"We dispatched multiple resources including rapid response vehicles, ambulance officers, three ambulance crews, our hazardous area response team (HART) and air ambulances from Essex and Herts Air Ambulance (EHAAT) and East Anglian Air Ambulance (EAAA).
"Multiple patients were treated at the scene.
"One patient was transported to Colchester General Hospital by land ambulance as a precaution for further treatment.
"A further two patients were also taken to Colchester General Hospital, both in a critical condition."
Essex and Herts Air Ambulance said in a statement: "We are currently attending an incident in the Clacton area.
"At this time three critical care teams are in attendance: two from Essex and Herts Air Ambulance and one from the East Anglian Air Ambulance.
"They are helping to provide critical care along with our partners from the East of England Ambulance Service Trust and other emergency services."
A spokesman for Clacton RNLI said:"We were called at 1.45 and launched our D class lifeboat and made our way to the pier. We were flagged down by beach patrol on a jet ski who directed us to shore towards the casualties."
They were then told to stand down as Essex Police, ambulance service and Tendring Council were on scene.
"We were later called due to concerns there was a fourth person missing in the water, but we were later stood down," he added.
Tendring District Council said on Twitter: "We are aware of a situation currently ongoing on a beach on Clacton seafront, our beach patrol team and the emergency services are attending to the scene."
Hedingham Buses announced on Twitter that it had diverted buses in the area:
"Due to a serious accident on Marine Parade West in Clacton, all services are diverting via Wash Lane in both directions.
"We are also expecting severe delays for the foreseeable future. Our sincerest apologies."
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