Civilian khakis, not uniform.
Really doesn't matter as on merchant ships which is what they are training to work as officers on, they will be wearing either regular civilian clothes, or at most khakis - ALL YEAR ROUND.
Further in the Navy, Khaki's in particular Service Khaki's are basically the equal to "Business Casual on Steroids" IMHO and an appropriate uniform that I see worn every day, year round at the Pentagon as well as the Navy Annex and office spaces in and around DC for all occasions except for days when there are ceremonies going on in that office like Change of Command, etc.
Also I though that as of circa 2007, the Navy phased out service blue and service white uniforms for Junior enlisted and replaced it with the new "Navy Service Uniform" which according to wikipedia is: "The Navy Service Uniform is a year-round service uniform to withstand day-to-day classroom and office-like environments where the service uniform is typically worn. It consists of a short-sleeve khaki shirt for males and a khaki weskit style blouse for females, made from a wash and wear 75% polyester, 25% wool blend, with permanent military creases, black trousers for males with beltless slacks for females and optional beltless skirt, and a black unisex garrison cap. Silver anodized-metal rank insignia is worn on shirt/blouse collars and cap. The service uniform will also include a black relaxed-fit Eisenhower-style jacket with a knit stand-up collar and epaulets, on which petty officers wear large, silver anodized-metal rate insignia. Those entitled to wear gold chevrons continue to wear gold chevrons on the large metal rate insignia on the jacket."
Finally relevant to this is the point that Merchant Mariners generally only wear uniforms when they are working which is when they are aboard ship and as such the following Naval convention regarding uniforms applies: "Working uniforms are worn when other uniforms may become unduly soiled or are otherwise inappropriate for the task. These are worn at sea, and in industrial environments ashore. In July, 2010, the new Navy Working Uniform and coveralls became the only authorized working uniforms." If of course you even are so inclined as to apply Military/Naval regulations and conventions to the Merchant Marine, something I'm not generally a big proponent of doing.