If you are a mechanical engineer and wish to get a job in the merchant navy, then you can do so by becoming a marine engineer using your mechanical engineering degree. 
Mechanical and marine engineering courses have several subjects in common and this makes it easier for a mechanical engineer to get an additional degree in marine and open new doors of opportunities.
A one year Graduate Marine Engineering (GME) course after mechanical engineering would give students the licence to enter merchant navy.
Eligibility criteria:

• Graduation in BE (Mechanical) Engineering / Naval Architecture with minimum marks of 50% in final year
• Must have minimum 50% marks in English language at 10th or 12th or in Degree Exam.
• Must not be more than 28 years old
• Should satisfy all health requirements that are necessary to join the merchant navy.

Some of the prominent institutes offering GME course are:

• Academy of Maritime Education & Training, Chennai
• Samundra Institute of Maritime Studies, Lonavala, Maharashtra
• Vels Academy of Maritime Education and Training, Chennai
• International Maritime Institute Ltd, Greater Noida

Hope this helps you with the complete guidance and knowledge for the same.