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](https://en.wikipedia.org/wiki/USS_Augusta_(CA-31)), which was occassionally used for transporting the President (both Roosevelt and Truman) during WW II. He was returning from the [Postdam conference](https://en.wikipedia.org/wiki/Potsdam_Conference), which ended in August 2 in Postdam, Occupied Germany. (by KillingTime)

From [the US Navy page for USS Augusta](http://www.history.navy.mil/research/histories/ship-histories/danfs/a/augusta-iv.html):

> ... 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 safe then as today, so it was common for people to cross the Atlantic using ship. (by JustAnotherDotNetDev)