M.Tech

Hello Phookan,
You may refer the link below for the list of colleges for M.Tech in marine engineering:
https://www.shiksha.com/engineering/colleges/m-tech-marine-engineering-colleges-india

Doing B.Tech and preparing for GATE, PSU and IES is a better option for core branches students, rather than to go with MBA.

Hello Karthik,
Please mention more on your question as it is not with more details about your M.Tech in civil engineering.
For admission into M.Tech programmes, you are advised to appear for GATE exam, which is for admission into IITs and other reputed colleges in India for M.Tech.

Yes, for sure.
You don't even need to opt for dual degree course, you can get CSE in non-integrated course too, I guess.

If you want to get into purely technical side, then B.Tech + M.tech is better. But, if you want to have knowledge about management too, for better progress and all round knowledge and success, then go for B.Tech + MBA. Both of them have good career prospects depending on your choice and interest. However, many people opt for B.tech + MBA to be an all-rounder.