Canterbury Crown Court heard how the boy was at a summer activity camp run by St Edmunds School and was taking part in a scheduled swim when he got into difficulties and struggled for over three minutes before becoming motionless in the water.
The lifeguards noticed he was in trouble and retrieved him. He regained consciousness after CPR but did develop pneumonitis as a result of the incident.
An investigation by the Health and Safety Executive (HSE) into the incident, which occurred on 1 August 2014, found that the lifeguards were not effectively managed and monitored to ensure that they were constantly vigilant.
It was also discovered that two out of the three lifeguards did not hold a current, in date lifeguard qualification.
St Edmunds School Canterbury, of St Thomas Hill, Canterbury, pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc Act 1974, and was fined £18,000 and ordered to pay costs of £9669.19.