There are ATMs in Port Blair, the capital of the Andaman and Nicobar Islands. There are two ATMs in Havelock Island as well of late. But you should know that there is no actual bank in Havelock. So there are times when the machines don’t have any cash left. The ATMs there are replenished with funds once every three to four days. You should carry sufficient cash with you to pay your bills as other islands do not have ATMs.
Apart from big hotels, resorts and some diving facilities, credit cards (MasterCard and Visa) are not widely accepted in the Andaman Islands. You will be required to pay surcharge and your passport and credit card will get photocopied for submitting to the bank. This is because all the transactions are done offline. Moreover, American Express cards are not accepted by many establishments.