Dear Shrikant
Shrikant yeah you can be able to do after the completion of your respective degree but that depends upon the vacancies and the written exam / interview in which you have to qualify and regarding the side by side then the answer is NO you can't be able to do the job in Indian Navy while pursuing your BTech degree , since this is a Full time Job but yaah if you are really interested to earn some money / stipend then in that point of view you can go through with any type of part time job or internship from any of the good Organisation which will definitely gonna help your extra curricular skills.
All the best

About IMU CET exam 

Conducting Authority :- Indian Maritime University

Examination Level :- National

Category :- Undergraduate Examination

Mode of Exam :- Online

Duration of Exam :- 3 hours

Other details on below given link :-

Https://engineering.careers360.com/articles/imu-cet