hey there,

As you mentioned you belong to commerce stream you can only join Indian Army (not navy) after 12th through NDA entry,or even later after graduation through CDS as far as officer cadre is concerned. Officers joining air force and navy need to be of science background.

For Navy, cadet should be a Naval wing NCC cadet also must have a B.Tech degree. No 3 year degrees like B.Sc, BA , B.com are allowed.

you can join the merchant navy course after 12th class from the commerce stream . Any Class X pass students or higher but from non-science background can join the Merchant Navy by enrolling for the GP Rating Course. Candidates should be physically and medically fit, as per Merchant Shipping rules.