noun
navigation officer; mate
A licensed officer responsible for navigating a ship; a professional maritime role.
彼は大型タンカーの航海士として働いている。
He works as a navigation officer on a large tanker.
航海士になるには海技士の資格が必要だ。
To become a navigation officer, you need a maritime officer's license.
航海 means 'voyage' or 'navigation' as an activity; 航海士 is the person who performs it professionally.
Compound of 航海 (navigation) + 士 (person with a certain qualification or profession).