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)

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