Hi, After 12th you can pursue either Bsc or B.E .
Some of the courses are:

• B.Sc. Nautical Science
• B.E. Marine Engineering
• B.E. Naval Architecture and Offshore Engineering
• B.E. Petroleum Engineering
• B.E. Mechanical Engineering
• B.E. Harbour & Ocean Engineering
• B.E. Civil Engineering
• B.E. Electrical & Electronics Engineering

Also You should have class 12 with Physics, Chemistry and Mathematics.
Admission is through an entrance followed by a screening test and a main written examination.

After clearing the test, you will have to give medical test as well.
Good Luck.