As I've learnt from the What are “prohibited / restricted areas” in the context of an Indian visa question, a normal tourist visa issued to a foreigner isn't valid for much (all?) of the Andaman islands.
Inspired in part by this question on Diving in the Andamans, I'm considering trying to work a side trip over there to do some scuba diving on a future trip to India. However, I now discover that my current visa wouldn't cover that.
So, as a foreigner in possession of an Indian visa marked "Not Valid for Prohibited / Restricted and Cantonment areas", how do I go about getting permission / extra visa / permit to visit the Andaman islands?