As you want to get the details of differences between Marine Engineering and Merchant Navy so here is the information of the same for you:

Merchant Navy was a group from WW2 who were a volunteer group of navy sailors and captains who helped get merchant goods to the sailors overseas during the war. (Food, clothing, letters, etc).

Marine Engineering is career path through which you can enter in Merchant Navy

In this field you have to do work with building boats and ships--and anything that has to do with water (marine) engineering.