Yes, Only right grades and the passion is not enough to get a career in merchant navy.

Your height should be or above 150 cm, else you will face a rejection.

And to know more about Physical Fitness and Medical Requirements to Join Merchant Navy you can refer below link-
(Source)

https://www.marineinsight.com/careers-2/physical-fitness-and-medical-requirements-to-join-merchant-navy/

Yes, this is one of the most important factor to get admission in Marine Engineering as the minimum height should be 150cms with correlated weight and proportionate chest with minimum 5-cm expansion.

Good Luck!!!