The 14-year-old was given CPR at the scene by a passing soldier after he was rescued alongside his friend, 12, by two "brave" teenage girls.
They had gotten into difficulty near Central Pier at 6pm on Monday, 18 July.
The 14-year-old is in a critical condition at Alder Hey Children’s Hospital, while the other boy is at Blackpool Victoria Hospital in a 'poorly' but not in a critical condition.
Police, fire crews, ambulance, air ambulance and lifeguards and lifeguards were all dispatched to the incident.
A cordon was quickly set up as large crowds watched on and a large section of the promenade was sectioned off.
Lancashire Police say the incident could have ended with "at least one fatality", once again showing the "dangers that the open water can present".
Superintendent Chris Hardy said: "I have no doubt that if it hadn’t been for the brave and swift actions of the two girls who helped pull these boys from the water, and the security staff who gave CPR, that we would have been dealing with at least one fatality.
“Over the last couple of weeks we have very sadly seen a number of youngsters across the country who have died after getting into difficulty in open water and a number of families left bereaved.
“As this very warm weather continues, at least in the short term, I would again plead with people to enjoy the beach but to respect the water and take care around swimming in the sea, especially if you aren’t prepared or are not a strong swimmer.”
Blackpool lifeboat crews were among those to respond with assistance also coming from coastguard’s in Lytham, Fleetwood and Knott’s End.
An RNLI spokesperson said: “Our RNLI volunteers were called out to reports of two teenagers in difficulty in the sea at Central Pier.
"The D-Class inshore lifeboat was just about to launch, when members of the public managed to get the two back to the safety of the beach.
"The lifeboat crew assisted in medical care with assistance from North West Ambulance Service NHS Trust."
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