Income tax rules if working on a cruise ship

Hello,

I need some information regarding income tax requirements for an Indian citizen working on the cruise ship.

He is a citizen of India, but works abroad on ships in the Caribbean, US etc. and stays in India about 2-3 months during a 12 month period.

Is he required to pay income tax in India?

Is he considered a NRI or is there a special seaman clause for tax purposes?

He does not specifically reside outside of the country as his home is the ship.

Thanks in advance for any help.