Naval Ranks are a little different, at least in western fleets, than ranks in other armed services
Here are the details to make your naval characters more realistic.
Note that after most of the First Term Enlisted skills listed in the character generation charts, a title is showns. This is the character's Rating. This is an integral part of a naval character's Rank in the navy, the character's "Rate".
For example, a Submarine Service character who chose Computer 2 as a first term Skill, would see "Sonarman" as a title. When the character is promoted to NCO rank ("Petty Officers" in Naval parlance) as per the Twilight rules, his rank becomes "Sonarman 3rd Class". If the character continued to the highest rank allowed, he'd be a "Senior Chief Sonarman".
Enlisted Ranks
Seaman (while there are multiple grades of Seaman, most characters will be promoted to a Rate and Rating within their first term in the Navy.
(Rating) Third Class
(Rating) Second Class
(Rating) First Class
Chief (Rating)
Senior Chief (Rating)
Officer Ranks
Ensign
Lieutenant (Junior Grade)
Lieutenant
Lt. Commander
Commander
Captain
Commodore