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 Potsdam conference, which ended on August 2 in Potsdam, Occupied Germany. (by KillingTime)
From the US Navy page for USS Augusta:
... 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 then were not as safe as today, so it was common for people to cross the Atlantic using ship. (by JustAnotherDotNetDev)