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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)

Planes were not safe then as today, so it was common for people to cross the Atlantic using ship. (by JustAnotherDotNetDev)