Megalodon Apocalypse — Eric S. Brown

The rains came and washed away the world of man. The waters covered the Earth. Everyday life became a struggle to stay alive for those who survived the floods. Pirates roamed the oceans preying on the weak. Admiral Campbell's vast armada, known as the Fleet, and the massive platform city of Dry Dock became the last holdouts of civilization. While mankind continued to make war with itself, a new evil was growing. Beneath the waves, life mutated in the wake of the great flood giving birth to the Megalodons and now the final battle for dominion over the Earth has come.