From what ive read in the books, like when the Chief was being breifed in an after action report by top brass, they wear the standard naval uniform. If they maintain the uniform standards of today, they will probably be wearing the black service uniform. Probably a more skintight version if most current sci-fi uniforms can be used as a template(haha). They cant be in uniform all the time, even if it was thier second skin as they say.