Diploma in Nautical Sceince is a 1 year course under the Indian Maritime University. Prospective deck cadets undergo a year long period of study followed by a sailing period of a minimum of 18 months.

Yes, in order to become a deck officer, you can do the DNS course. I would also  recommend you to check the Directorate General of Shipping and the Indian Maritime University websites for institute approvals and  more course details.