Hello Pratap, personally I found Nikhil from Blue Corals Dive in Havelock Island a thorough professional and he has a very good team with him. Plus, he gives great discount if you go during off-season. Here's the address - Blue Corals Dive. Beach no 5, Govind Nagar, Havelock, Andaman and Nicobar Islands - 744211. The website is http://www.bluecoralsdive.com/ Say hello to him from my side if you do take a course under him :D
Some of the best places for scuba diving in Andaman -
Elephant beach -Havelock Island
North Bay-Port Blair
Govindnagar Beach-Havelock Island
You can also book scuba diving after reaching Andaman.
Hope it helps you.
There are many organizations like 1. India Scuba Explorers
2. Scubalov.in Dive Resort
3. Eco Diver India Pvt. Ltd.
4. DIVEIndia Scuba & Resorts Pvt. Ltd, Havelock Island, Andamas
5. Experience Andamans You can go through there online website or call them or you can book diving after reaching there. Happy journey my friend :)