This is the one thing that remains rather difficult to explain about Colm Meaney's constant rank identifier changes in the various shows. Dialogue basically agrees that he always was a noncom and never attended the Academy - so how did he become the Tactical Officer of the Rutledge?
I have heard that the "Diving Officer" position on U.S. Submarines can be held by a non-com...in some cases the "Chief of the Boat". So it could be a case of it being a description of the position
rather than a job requirement.