Captain is 0-6 in the Navy.....equivalent to an Army/Marine Corps/Air Force Col. His next promotion is to Rear Admiral!
Go see, he’s not an O-6.
It gets confusing. Captain is both a paygrade (O-6) and a title. Any CO of a ship is referred to as "Captain" regardless of their paygrade. As such, CDR Zettle would have been addressed as "Captain" by his crew but his rank was Commander, or O-5.