Two teenagers are in hospital after going into the sea to retrieve a football after playing on a beach.
Rhyl RNLI and the coastguard were called to the town beach, near the harbour entrance, at about 22:30 BST on Saturday after numerous 999 calls.
The RNLI said one boy had gone into the water to retrieve a ball while the other went in to help after the first person had "got into difficulties".
One boy showed signs of hypothermia, lapsing in and out of consciousness.
The other boy was very cold and both were taken by ambulance to Glan Clwyd hospital after receiving treatment from lifeboat and coastguard crews.
"As they hit the water, one of them succumbed to cold water shock and had to be dragged out by the others," said Paul Frost, duty coxswain for Rhyl RNLI.
"Another person appeared to be hypothermic.
"One lad was unconscious and he was given oxygen and casualty care. The second person was cold and shivering and he was warmed up," he added.
The RNLI warned that "even though the air temperature is warm, the water is still cold" even at this time of year.