The little boy had been dragged under the water by one of the dogs after being bitten. His brave granny pulled the dog off, but the child had already sustained around 20 wounds to his chest, back and arms.
The dogs ran from the beach with their owner, who is being hunted by the police.
The youngster is now in hospital in Chelmsford, Essex, and has undergone two rounds of surgery.
The police told reporters: 'The grandmother had to pull the dog from the boy for fear it was going to drown the child or bite him to death. It must have been a terrifying ordeal that has left the lad with horrific injuries both physical and emotional I am sure.'