You have two options if you want to work in deck jobs in Merchant Navy. One is that you can do a B. Sc. in Nautical Science and directly get a job as Deck/ Navigating Officer/ 3rd Mate (subject to your qualifying the competency examination conducted by the Ministry of Surface Transport, Government of India). The B. Sc. in Nautical Science is offered at T. S. Chanakya under University of Mumbai. For admission, you have to clear the IIT-JEE examination conducted by the IITs.

The second option is a direct recruitment as a deck cadet with a shipping company like the Shipping Corporation of India. Shipping companies do not really sponsor you. They recruit you as a deck cadet and train you with 3 months Pre-Sea training course followed by 27 months on-board training course. You have to pay for the Pre-Sea training course. However, you are paid a stipend while taking the on-board training course. So, no shipping company sponsors you as such. At the end of the training course, you have to sit for the 2nd Mate Competency Certificate examination conducted by the Ministry of Surface Transport. If you qualify, you can get a lucrative job offer from the shipping company. To be eligible for deck cadet course in a shipping company, you must have passed 10+2 with Physics, Chemistry and Mathematics with at least 60% marks in aggregate.

For application, look for advertisements of Shipping Corporation of India and other shipping companies in the leading dailies in India.