Hi , as per your marks I would like to tell you that 50% marks are too less to get you a seat in any college for nautical science as The minimum eligibility to pursue the course is that  you must have  passed 10+2 in science stream with Physics, Chemistry, and Maths as the main subjects and the minimum score of 60%. So according to that you don't even qualify the criteria in order to appear in entrance exams like IMUCET, CUSAT CAT, etc. But don't feel bad you can for many other options as well.

Hope this helps