Colourful fishes, Underwater world, Coral Life and much more in Agatti Islands and many other smaller islands in Lakshadweep, India.
Technically part of India, but it is away from India mainland.
It is a much bigger experience than going to Andaman & Nicobar Islands or Maldvies.
Unlike others, there are not many 
One need to go either by Cruise or only very few limited flights going there.
Again you need a booked accommodation if you need to get in through the flight, and not so many options for hotels.
Good option is always to go with the cruise from Kochi by Kavaratti Ship and Samudaram 