Mumbai

If you are weak then you can take classes regarding its subjects.

Please check the link provided for the college suggestion according to your rank and branch specified:
https://www.shiksha.com/b-tech/resources/mhcet-college-predictor

For getting a govt. college for BDS, you need at-least 450. However, you can get admission in a private college.

Best B.sc(IT) Colleges in Mumbai are:-
Rupareal
Somaiya
St. Xaveries
Khalsa
S.I.E.S

You may refer to the following link for a list of top mechanical engineering degree colleges in Mumbai: https://www.shiksha.com/b-tech/ranking/top-engineering-colleges-in-mumbai-all/44-2-0-10224-0

The fee structure depends from college to college. Colleges providing the best infrastructure and good amount of facilities will demand more fees as compared to that of the other colleges providing low infrastructure facilities.

I think mechanical engineering from Heritage Institute of Technology is a better option but marine engineering takes long. If you go for marine engineering, you go through a long process like interviews, medical checkups. After doing your B.Tech from Heritage, I think you should have to prepare for GATE for M.Tech or you do one more thing which is to start your GATE coaching in the very 1st-semester to last. After doing 4- years of preparation, you will get IIT for M.Tech easily.