The USS Minneapolis-St. Paul was leaving Plymouth harbor accompanied by police boats in rough weather, police said. The four sailors were tied to the vessel with ropes — a routine precaution — but were knocked into the water by surging waves.
The police boats — small inflatable dinghies with outboard motors — picked up all four, said Baxter Provan, a spokesman for the Devon and Cornwall police.
Lt. Chris Servello, a spokesman for the U.S. Sixth Fleet in Naples, Italy, said the four men were taken to hospital in Plymouth, where two were pronounced dead. The two survivors were treated for minor injuries and discharged, he said. No one else was injured and all the other sailors have been accounted for.
Authorities received a request for help just before 1 p.m., near the large concrete breakwater barrier that rings Plymouth harbor. The British Coast Guard dispatched a search and rescue helicopter and a lifeboat to the scene, but police had already plucked the men from the water.
Sean Brooks, a Coast Guard officer, said rescuers initially only saw two sailors tied to the vessel's hull with ropes.
"Because of the violent weather, they were frequently plunged below the waves," he said. "It then transpired that there were already two other guys in the water."
The two men were given CPR as they were carried ashore, but it was not enough to save them. Their names have been withheld pending notification of next-of-kin.
The Devon and Cornwall police are investigating the deaths together with the military.
Provan said the two survivors would probably not have been rescued as quickly had it not been for the police escort, which was assigned to the submarine because the higher security accorded to American vessels.
"You could say it was lucky they were there," he said.
Servello said the submarine had just completed a weeklong layover in Plymouth and was heading out to sea for routine duties.
The nuclear-powered attack submarine, which is based in Norfolk, Va., has a crew of 110 and can reach speeds exceeding 30 miles an hour. Its weapons include torpedoes and Tomahawk cruise missiles.
The Sixth Fleet is the operational arm of U.S. Naval Forces Europe and is primarily responsible for the Mediterranean and western and central Africa.