Rather peculiar question regarding energy and an underwater environment

[Sweyn]

This question is very odd... to explain, it is for a post-apocalyptic novel where the surface has become uninhabitable and the population lives under water. Quite a simple question none-the-less... what would be the most abundant and most effective source of energy? If even touching the surface of the earth was completely out due to some disaster maybe fallout or some biological warfare... maybe the surface above the sea would be okay as long as it was far away from the coast. And would everything become eventually stable or would it be a very difficult struggle that would eventually get worse until life would cease? Also, note that the population is extremely low. With a maximum of 8% of the original population surviving. Any help would be much appreciated. Thanks.

[heathfield]

Geo-thermal energy?

[Clachair]

Assuming the planetary orbits remain much the same, tidal flows could provide mechanical and/or electrical energy if you can engineer a way to harness it.