It's a topic which would be split by the type of ship/shipping company. Aboard passenger carrying steamers and liners there was more similarity to the Royal Navy, probably because certificated officers were frequently also officers in the Royal Navy Reserve and uniform would be worn by men with Masters or Mates certificates, engineers would normally wear overalls for their working watches but would have uniforms to wear in public or in the officers saloon. Aboard deep sea tramp steamers (carrying general cargo to distant ports) things were usually informal and I gather that often the Master might be the only man aboard to have a uniform. I dont believe that officers aboard Home Trade vessels would have worn uniform and havent seen photos of them being worn although I am certain that railway company steamer officers would be uniformed.
There is little written, you may have success with Liverpool and the National Maritime Museum at Grrenwich.
Photos can often be seen for sale on ebay of Merchant Service/Mercantile Marine ships officers in the pre-Titanic era.
