Was ”captain” a rank or a job?

It could be both. The commanding officer of any ship, such as a submarine, was referred to as the captain regardless of his rank. Captain is also a naval rank just below admiral. Commanding officers of large ships may have been captain of that ship and also hold the rank of captain. In WW2, captains (commanding officers) of submarines almost never held the rank of captain. They were usually commanders or lieutenant commanders. However, they were always addressed as captain since they were the commanding officers.