I had been to Andaman & Nicobar Islands last year..
Flights tickets are available for cheaper rates, if you book for both to and fro travel.
However, rates depends on the season also.
By flight you can reach Portblair within one and half hours from Chennai.
I think there are flights available from Kolkatta also.
As Ritesh ji said, the ship journey is bit difficult. If you are in a holiday mood, you may not be enjoying it. Make sure the seats are booked in Deluxe class.
For Nicobar Islands, you may need "Tribal Pass" also, which can be obtained from Deputy Commissioner office. Am not sure whether the tribal pass is necessary for tourists.
However, in Nicobar conditions are worst after the tsunami. You may have to book boats to go to Indira point, as the roads are totally destroyed during tsunami. anyway you can visit associate group of islands..
I had a contact number one guide from Port Blair, which I misplaced some where.
I will send it later..
wishing you a happy journey...
Manudev K Madhavan
Junior Research Fellow
Systematic & Floristic Lab,
Department of Botany,
Centre for Postgraduate Studies & Research
St. Joseph's College, Devagiri
Kozhikode- 673 008