Yes, you can join the merchant navy course after 12th class from the commerce stream. To get jobs in merchant navy you have to pass one of the following courses:

Courses:
6 months Pre-Sea course for General Purpose Rating
6 months Pre-Sea course for Saloon Rating OR Certificate Course in Maritime Catering (CCMC)
One year Pre-Sea Diploma in Nautical Science leading to B.Sc Nautical Science
3-Year B.Sc. Degree in Nautical Science
2-1/2 years course for Marine Engineering under Alternate Training Scheme (ATS)
4-Year Degree course in Marine Engineering
3 year Polyvalent (Dual certification) course leading to B.Sc (Maritime Science) Degree

Eligibility for Merchant navy:
Merchant Navy can be joined after 10th as seafarer after qualifying pre sea courses
After 12th MPC (only) you can join as an officer
You may also join through various degree or diploma courses in mainly two specialized fields
i. Nautical (Deck)
ii. Marine engineering.

The age limit is 16 - 25 years
Unmarried Female and Male candidates are eligible.
You must have a fit and healthy body

Minimum Height: 167 Cms.
Eye Sight: 6/6

Chest: 77 Cms. in normal mode
Chest: 82 cm in expanding mode

Average Minimum Weight
50 Kgs