Hello student yes you're eligible for Merchant Navy as NIOS is a recognized board and student passed from NIOS are eligible to pursue Merchant Navy. However eligibility criteria vary from college to college so you need to fulfill the eligibility criteria of the respective college in which you're willing to pursue Merchant Navy.

ELIGIBILITY - Candidates must passed or appearing class 12 in physics chemistry and Mathematics with minimum 60 percent marks from a recognized board.

I request you to please help me with your preferable college so that I can check and provide you the exact information about admission process and eligibility requirements.

Feel free to comment if you've any doubt

Good luck