Four children seen "up to their necks in water and waving for help" had to be rescued by the coastguard.
The dramatic scenes at the onset of the Bank Holiday weekend unfolded in Southend, Essex.
The children were returning from Mulberry Harbour near Thorpe Bay when they got into difficulty at about 3.05pm on Friday.
They were rescued by the coastguard in a hovercraft and taken to hospital with onset hypothermia.
They are now believed to be back at home.
The coastguard has warned against walking to structures in low tide, and gravely added: "Cold water can be silent and deadly. Even on the warmest of days. It kills."
The rescue body added that 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," the statement said.
After the children were rescued and brought to shore, the coastguard found they needed "medical attention due to suffering the onset of hypothermia".
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."
