Prince William took part in the rescue of Russian sailors whose cargo ship sank.
Prince William co-piloted a helicopter which rescued two crew members.
Five people remain missing after the cargo ship sent a mayday call .
The rescue effort was described as frantic after gale-force winds battered the cargo ship.
Prince William co-piloted a helicopter which rescued two people after a cargo ship sank in a storm off the Welsh coast, leaving one man dead while two were winched to safety by helicopter, officials said.
A sea search was underway on Sunday for five Russian crew members.
The two rescued members of the crew, who were transporting 3,000 tonnes of limestone from Colwyn Bay in Wales to Cowes on the Isle of Wight, were found clinging to lifeboats in the water.
They were winched up by a Sea King helicopter co-piloted by Prince William, who works at RAF Valley in north Wales, the Ministry of Defence said.
Another crew member was found in the water later in the morning but has been confirmed as dead, the Maritime and Coastguard Agency said.
"There were eight persons on board, all Russians," coastguard official Ray Carson told the BBC.
The 81-metre (265-foot) vessel, the Swanland, sent out a distress signal around 2:00 am from the Irish Sea when it was hit by a huge wave which ruptured its hull, coastguard officials said.
Speaking from Holyhead in north Wales, Carson said: "The two men recovered from the water were brought here before going to the hospital. I think they are OK and are just suffering from shock.
"In broken English and through drawing a diagram, the second officer told us the ship was hit by an enormous wave. It rolled the ship and it broke its back. He said this led to a catastrophic failure of the vessel."
Several lifeboats and helicopters continued the search for the missing sailors into Sunday afternoon, officials said.
Confirming Prince William's participation in the overnight operation, an MoDefence spokesman said: "The Duke of Cambridge was a co-pilot on one of the aircraft involved. He is now back at base."
The ship was owned by a Britain-based company, Torbulk, and was carrying the flag of the Cook Islands, according to shipping websites and media reports.
In August 2010, the Swanland suffered engine failure in high winds and nearly went aground on the rocks off Cornwall in southwest England.