"Ecco the Dolphin" by Appaloosa Interactive. It is set in a peaceful future time in which humans and dolphin live in a peaceful perfect civilization. Which is now threatened with destruction by a invading force know as the 'foe'. The foe use time travel to destroy the future of Earth. You take on the role of ecco the dolphin who is only being that can save the future of earth. As ecco you must swim through huge underwater levels, solving puzzles and meeting mysterious underwater creatures with the eventual aim of saving Earth.