Diploma

Please refer to the link provided below for the list of colleges which offer admission into marine engineering courses in India.
Here is the link:
https://www.shiksha.com/b-tech/colleges/b-tech-in-marine-engineering-colleges-india

For a bright future, you should definitely follow a career wherein your interests lie. A Diploma in radiology is a good option. For now, you should study hard and devote a lot of time to practice as you will need this experience when you enter the industry as a fresher. Although you won't get hands

If you go for better institute then the fee may be higher but they will give you better knowledge.

Yes, you should be able to pursue B.Sc in radiotherapy after your Diploma provided you meet the minimum requirements to gain admission in B.Sc.
B.Sc Radiotherapy eligibility:
1) 10+2 (Medical) or equivalent with 55% marks.
2) Few institutes conduct entrance exam for admission.

Yes, you can do B.Tech in Computer Science after completing Diploma in Mechanical Engineering if you have a decent grade in your Diploma studies.