Tax-ability is dependent on your residency status. If you are deemed resident Indian, you have to pay tax. If you are deemed non-resident you are not liable to pay tax.
You mention that you have NRE, but haven't mentioned your residency status in India for tax purposes.
Simplistically if you are outside of India for more than 182 days you are non-resident for tax purposes. However in case of Merchant Navy, there are other factors like who owns the Ship, i.e. is it under Indian management, etc that needs to be used to determine if your residency status is RNOR [Resident but Not ordinary Resident], in which case you need to pay tax.
Please consult with a qualified CA for your specific case.
Also note any interest or other income that gets generated in India is taxable in India.