Putting together an answer based on comments from several users, before this gets closed. Hoping it might be useful as quick reference of future readers.
He was on the USS Augusta, which was occassionally used for transporting the President (both Roosevelt and Truman) during WW II. He was returning from the Postdam conference, which ended in August 2 in Postdam, Occupied Germany. (by KillingTime)
... until 7 July, when President Harry S. Truman, Secretary of State James F. Byrnes, and Fleet Admiral William D. Leahy boarded her, and she stood out for Antwerp, Belgium, to carry her distinguished passengers on the first leg of their voyage to the Potsdam Conference
On 2 August, she embarked her distinguished passengers again, and received another visit from King George VI. Harry Truman was on the ship when he got the news that Hiroshima had been bombed by an atomic bomb. The ship then sailed for the United States, arriving at Newport on 7 August to disembark the President.
Planes were not as safe then as today, so it was common for people to cross the Atlantic using ship. (by JustAnotherDotNetDev)