Hello student both Are two different fields, on comparing merchants navy graduate with Mbbs graduate the salary mbbs graduates are more. They earn around 12-15LPA (starting). But I'll suggest you that choosing your career decision on the basis of salary is not good go with your interest and in which you can do better that'll help you in future and you can earn enough in that instead of working day and night to compete others.

Feel free to comment if you have any doubt
Good luck