While a score of 50% in your Class 12 PCM might make it slightly more challenging, it doesn't entirely rule out your chances of joining the merchant navy. The eligibility criteria can vary depending on the specific course and the maritime training institute you are applying to.

For certain entry-level positions, particularly for ratings (general purpose or engine), some institutes might consider candidates with an aggregate PCM score of 50%. These roles often involve basic seafaring duties and can be a starting point for a career at sea. However, for officer-level programs like a Bachelor of Science (BSc) in Nautical Science or Bachelor of Technology (B.Tech) in Marine Engineering, the minimum PCM aggregate required by many reputed institutes and for appearing in entrance exams like the Indian Maritime University Common Entrance Test (IMU CET) is often 60%.

Different courses in the merchant navy have varying eligibility criteria. For example, a Diploma in Nautical Science might have slightly lower percentage requirements compared to a full-fledged B.Tech degree. Investigate the specific requirements for the courses you are interested in.

For many officer-level courses, you'll need to clear entrance exams like IMU CET. While the eligibility to sit for these exams might have a PCM percentage requirement (often 60%), performing exceptionally well in the entrance exam could potentially compensate for a slightly lower board score in some cases.

In conclusion, while your 50% in PCM might limit your options for direct entry into the most sought-after officer-level programs in top institutes, it's still worth exploring various courses and institutes. Focus on researching specific eligibility criteria, preparing well for any required entrance exams, and considering alternative entry routes if necessary. Good luck with your endeavors!