A somewhat formal or occupational term for a sailor or mariner. In modern casual speech, 船員 (せんいん) or 船乗り (ふなのり) are more common.
When he was young, he traveled the world as a seaman.
Compound of 海 (sea) and 士 (man, warrior, person of a certain occupation). The exact historical derivation is uncertain, but it follows a pattern of occupational terms using 士.