Physics

Yes, you can apply for joining Merchant Navy, but only 10+2 Science in PCM is not the only demand for joining Merchant Navy. For it, you should also fulfill some other eligibility criteria for joining Merchant navy. So I have put here the eligibility criteria for it. Educational qualification: The minimum eligibility for courses in Nautical Science and Marine engineering is 10+2 or an equivalent examination with physics, chemistry and mathematics. He should also pass a medical fitness test for sea service under standard norms and with eyesight 6/6 with no color blindness. Candidates wishing to enter the navigational and engineering fields of Merchant Navy need to complete a Bachelor's degree in Nautical Science or Marine engineering, before they can gain admission to the field.

It's great that you know what you want, now there is this point which makes sense, but isn't said very often. Scoring 90% in board is easy, as the papers are made (80-90%) out of previous five year questions. So, if you can I'll suggest you buy previous five year book now and don't wait for next Jan for it. There are books that covers question asked chapter wise, go for it. Before and after getting a chapter done in class, coaching, yourself which ever place you learn and can say I studied it properly, come back home take day in weekend and see the questions and figure out like 10-15 points or theorem or formula that the questions in Exam are based on. I don't remember the exact chapter number but I guess its 13-14 chapters so, around 170-180 points. That's is the key for all subject all boards. Now if you don't have the best resource, happen all the time, go search 'NPTEL' the professors of IITs teach physics chemistry and Maths there. It's simple and extremely good. And, since you have declared that your physics is bad, Don't Practice, try to analyses why is it happening and Google question that comes to your mind for 1 week, you'll see the difference.