If you're looking for an undergraduate or post graduate course in Merchant Navy there are two options:
On board job (undergraduate course: B.Sc(Nautical Science), Diploma in Nautical Science, B.Tech(Marine Engineering) : for which you have to be an intermediate pass out with Physics, Chemistry & Maths as subjects. (for eligibility you can go to WELCOME TO INDIAN MARITIME UNIVERSITY and look for Academic Brochure.)
Shore based job (logistics & Port Management ; MBA, BBA)- you can join these courses even if have not studied PCM in intermediate or undergraduate level, but it's not Merchant Navy, it's basically management part of the Merchant Navy. (Again for eligibility you look for the Academic Brochure of Indian Maritime University).
PS: there are some more courses which are related to Merchant Navy, like- GP Ratings, Steward Department etc.(I guess it's not your concern)