For the most part civillian officer's wear either coveralls (sometimes called a boiler suit) or regular civilian clothes as long as those clothes don't interfere with their jobs. exception: Cruise shiups and passenger liner Crew will usually wear a uniform derived from a traditional uniform (most often Summer Whites)
The differences in naval and merchant uniforms lie largely in the details of insignia, hat devices, buttons (on dress uniforms); for example, a US naval officer's insignia of rank (gold stripes on a shoulder board or on the cuff of the coat) will be surmounted by a specialty insignia. US Navy Line Officers have a five pointed star superior to the stripes. US Merchant Marine Deck officers holding positions similar to those of Navy Line Officers will have a fouled anchor superior to the stripes.
US Navy personnel render a "courtesy" salute to US Merchant Marine Officers (of an apparantly superior rank) in uniform.
An interesting tradition related to that fouled anchor on the Merchant Marine officer's shoulder boards and sleeve: one end of the rope fouling the anchor is secured to the anchor and is unseen; the "free" end (called the "bitter end") of the rope is visible at the bottom of the anchor. That bitter end, whether on shoulder boards or embroidered or sewn onto a sleeve, must point aft when worn, as if the fouling rope is trailing after the wearer. The insignia is worn with the bitter end pointing forward only on the uniform of a mariner being buried, whether at sea or ashore (a more familiar - perhaps - parallel is the inverted stirrups of the riderless horse in a formal military funeral procession.)