A question for Oda-sensei. In the Marines, there are some people who wear uniforms and some people who wear normal clothes. Is theright to wear regular clothes something that is decided by rank, or is it all up to personal preference?
O: Well. If it's preference, it's preference. If it's decided, it's decided. First of all… When you become a sailor, you are giventhe basic Marine uniform and cap (as shown). This is a very joyous occasion where you usually call home to tell your parents.Everyone Private First Class [Ittouhei] and below wears these. Jango and Fullbody are exceptions. When you become Corporal [Gochou] and above, you can choose from a number of uniforms. Also, personal clothes are allowed. However, if you want to appear sailor-like, you must exert some self-control. When you get to Warrant Officer [Juni], you are allowed to wear the word "Justice". For the Major [Shousa] class and up, the Justice coat and suit is preferred attire. (For more info regarding Marine ranks, consult volume 8.)
… Volume 24, page 26, SBS. Hope that helps. =D