I first heard about temples being under water back in 1982. I did see the National Institute of Oceanography (NIO) office during 2014, but did not go in as I was with people whose interest was more in picnicking rather than ancient history.
That said, the temples were said to have been built during the reign of the Pallava king, Narasimhavarman I (630–668 AD) and his successor Rajasimhavarman. The Pallava dynasty occupied what is now southern Andhra Pradesh, and northern Tamil Nadu. The Mamallapuram (Mahabalipuram) site is in Tamil Nadu.
The origins of the Pallava dynasty are a bit of a mystery. They apparently used Sanskrit as their court language, as opposed to Tamil. There are many theories of where the Pallavas came from, but my own theory is that they are of Telegu (Andhra Pradesh) origin.
The Pallavas had a great impact on South East Asia,which is largely underplayed in the literature.
The architecture of the Bujang Valley (Serpent Valley), in Kedah, Malaysia bears a remarkable resemblance to the architecture of Mamallapuram (Mahabalipuram).
The Pallava script was used in the Bujang Valley and also in Cherok Tok Kun, Penang, Malaysia . It is also to be found in Takua Pa, Phang Nga province, Thailand.
A lot of the Pallava kings have the name (or title) “varman” e.g. Mahendravarman I, Narasimhavarman I, Mahendravarman II, Paramesvaravarman I, etc
Interestingly, some South East Asian kings also have the name “varman”. One example is Sangrama Vijayatunggavarman, a king from Kadaram (present day Kedah, Malaysia, where the Bujang Valley is located) [en.wikipedia.org] . This particular king was captured when the Chola Empire from Tamil Nadu, India, attacked Kadaram (Kedah), Malaysia in 1025 AD.
Another South East Asian king who has the name “varman” is Suryavarman I of the Khmer Empire (Cambodia).[en.wikipedia.org]