A Captain is either a military rank or refers to the person in charge of a ship (who may or may not also hold the rank of Captain).
In the 18th and 19th century the Royal Navy rank of “Captain” was correctly called “Post-Captain”, to distinguish it from the commander of a vessel (addressed as Captain regardless of actual rank). To “make post” was an important step when rising through the ranks of the Navy as from then on, promotion happened on a strict basis of seniority. Once a post-captain, an officer would always be addressed as Captain (not Post-Captain), even when not currently in command of a ship. The name supposedly stems from the tradition of “posting” a new Captain’s name in the London Gazette, but that causal relation may be apocryphal. The Post-Captain rating no longer exists.
A person in charge of a ship is technically a commander, but is traditionally addressed as “Captain”, independent of their actual rank. In the Royal Navy and the East Iindia Company, in the 18th and 19th century, a rank of Lieutenant was the prerequisite for being put in charge of a ship (a famous example would be
Captain Lieutenant Bligh). A lieutenant would commonly have to prove himself in command of a smaller ship before being promoted to Post-Captain and put in charge of a ship permanently.
A captain is in charge of a ship, so the final decision on every issue is his. Most captains would have been at sea from a young age and would be expected to understand every aspect of seamanship, although they wouldn’t be expected to perform most tasks themselves. A captain was the highest authority aboard in many matters, including judicial and religious; as such, as captain could, f.ex. pronounce sentences or marry a couple.
Aboard the Obra Dinn
The Obra Dinn’s Captain was an Englishman called Robert Witterel. He was married to Abigail Hoscut Witterel, the sister of his friend, brother-in-law, and first mate, William Hoscut. His exact naval career is unknown, but it's likely that he received his commission at least 20 years ago. As traditional for Captains, Witterel was the last person alive aboard his ship, although not in the usual sense.