The teenager grabbed the woman as she was going under, before she was transferred to the St Peter Port lifeboat, Spirit of Guernsey, whose crew had already pulled three others – two men and a woman – from the water on Friday night.
He feared they would discover bodies.
Tom was out on a Rib with his father, who is a crewman on the lifeboat, testing the boat ahead of the teenager's birthday party, which was planned for Saturday.
They were between Guernsey and Herm when his father's bleeper went off with a red alert, the highest alarm that can be given.
Tom said they listened in to Guernsey Coastguard on VHF Channel 20.
'We heard them talking to the lifeboat, which was out. We heard that one of the marks was close to us so we headed over there.'
Just the two masts of the schooner could be seen above the waves.
'There were reports of the yacht sinking and we could see the masts,' Tom said.
'We phoned and confirmed the boat had sunk and called Channel 20, to speak to the harbour. We saw the lifeboat in the distance.'
Tom said they began searching for the sailors, looking on the horizon, but were worried they would find bodies.