Hello Aspirant,

You are surely eligible for pursuing ocean engineering, but right now when you are already a civil engineer it won't be a wise decision to switch to ocean engineering from civil. 

If you really desire to go for marine engineering now, you have to study this course for another four years, which means your B.Tech (Civil) degree will be of no value after that, but in that case, sit for GATE exam.

Only mechanical engineers are advised to join this field of merchant navy or ocean engineering.

Personally, I  advise you not to switch to marine after graduation in civil. As this would be a real loss of your four years graduation course.

Hope this helps.