A person must choose his career that suits his taste, aptitude, physical ability, mental ability etc. If the career selected is not satisfying, it could turn out to be a disaster. The job at sea demands physical labour, alertness, intelligence, high professional knowledge, ability to work under severe stress etc. So speak to yourself and understand your aptitude. Very few get an opportunity to pursue a career at sea and therefore, the general public has limited knowledge about it.
It will be in your interest to fully understand the career and life at sea before joining any pre-sea training courses. The literature available in the market on the subject may not project the right picture about the merchant navy.