He went back into the water, this time accompanied by a surfer with a board.
"He was on his back and kicking too, but he was going under," he added.
"He disappeared 10 or 15 seconds before I got to him but I managed to grab hold of hair, then his hand, and pulled him back to the surface. He was crying by the time we got him on the surfboard, which I thought was a good sign."
Mr Andrews, who was still emotional and near exhaustion an hour after the rescue, said his 17 years in the military had stood him in good stead.
"I told myself to keep calm and stay focused," he said.
"I wouldn't have been any use to anyone if I hadn't kept it together."
The two boys were checked by a St John crew and did not require hospital treatment.