navigation officer; mate
A licensed officer responsible for navigating a ship; a professional maritime role.
He works as a navigation officer on a large tanker.
To become a navigation officer, you need a maritime officer's license.
航海士 is the standard term for a ship's navigating officer, while 運転士 is a broader, less common word for a ship's mate or officer in general.
Compound of 航海 (navigation) + 士 (person with a certain qualification or profession).