B.Sc

Oceanography courses in India are also referred as marine courses in India, mostly because they deal with the study of the sea. Oceanography is defined as a branch of science that deals with the physical and biological properties and phenomena of the oceans. Oceanography professionals are known as oceanographers.

No, B.Sc nursing is not offered in Allahabad University. Refer the link to know about the courses offered:
https://www.shiksha.com/university/allahabad-university-au-3574/courses/bsc-bc

Year of your +2 passing does not matter to get admission in B.Sc+B.Ed. For B.Sc, you are eligible. For B.Ed, you have to write an entrance exam.

No, it is not possible. For admission in 2nd-year of engineering you must have a B.Sc degree.

IIT JAM is not a course, it's an examination for pursuing M.Sc. From IIT, for more details check the link below:
https://www.shiksha.com/science/iit-jam-exam

Minimum marks required in 12th are 50% for admission in BSc (Hons.) in forensic science.
Check the detailed criteria here- https://www.shiksha.com/science/course/b-sc-hons-in-forensic-science-bundelkhand-university-uttar-pradesh-other-176584

Yes, you can join merchant navy after B.Sc in computer science. For more details, check out the link below:
https://www.shiksha.com/careers/merchant-navy-officer-77

Yes, you can. As per the eligibility criteria, you should clear your 10+2 or equivalent examination in Science stream with at least 50% marks in mathematics subject. Admissions are based on the entrance exam and merit. To check the colleges in India that offer B.Sc mathematics, please see here-
https://www.shiksha.com/science/colleges/b-sc-mathematics-colleges-india