To join merchant navy after mechanical engineering one has to do 1 year GME (Graduate Marine Engineer) course. One has to apply for IMU-CET (Indian Maritime University)  to get admission in a recognized college.

To know in detail you can refer below link

IMU Details

Merchant Navy is the backbone of international trade, carrying cargo all over the world. The merchant ships employ a huge number of workers to carry and deliver goods from one country to another. Without the merchant navy, much of the import-export business would come to a grinding halt.

A career in the merchant navy is considered a glamorous job, especially by the people who have been bitten by the travel bug. It offers an opportunity to visit new and exotic places all around the globe. Merchant Navy career is not only monetarily rewarding but also satisfying and a quite challenging.

All the best for your career!!!