USS Gedney - Coastguard

Looking for any info about a relative. He was employed by the US coastguards.
What is a ship manifest?
He was Leonard Howes b.1884 Rushden, Northampton Went to America on board Laurentic from Liverpool arriving Jan.1910.
He was classed as a hydrographer. Would anyone have any info of what he would have been doing.
He became a naturalised American in 1911.
Thank you,