Hey , It seems a little difficult or you can say negligible chances to get the top 5 IMUs . But there is also some chances to get some govt colleges . I mentioned these govt college below :- 

• Euro tech maritime academy , Cochin.

• Mangalore Marine college , Mangalore.

These two colleges are for marine engineering. For B.tech in naval architecture and ocean engineering there is no chance to get govt colleges according to your rank.

Hope it helps you.