Four children had to be rescued after they were seen “up to their necks in water and waving for help”.
They were on their way back from Mulberry Harbour near Thorpe Bay in Southend when they got into difficulty at about 15:05 BST on Friday.
The children were rescued by HM Coastguard Southend and taken to hospital with onset hypothermia. They are believed to be back at home.
The coastguard has warned against walking to structures in low tide.
In a Facebook post, the coastguard said the children “attempted to make their way back to shore but the water was too deep and caught them unawares and at one point were observed up to their necks in water and waving for help.”
“Do not walk out to any manmade hazards or structures at low tide you may see in the ‘near or far’ distance off Southend, you will put your and your families lives at risk.”
After the children were rescued and brought to shore, the coastguard found they needed “medical attention due to suffering the onset of hypothermia”.
It said the East of England Ambulance Service were called and took them to hospital.
The post added: “We wish the four casualties a quick recovery, who we understand are now back home.”