You look out through the porthole into the endless blue that one was London's west end. Now it is a sea with fish drifting through the old, submerged houses of the west end. Once there would be crowds people bustling through the streets but now the streets are empty except for a few crustaceans scuttling across the broken paving stones. A giant squid swam past the porthole and harpoons were fired into its side. Finally a catch.
You are a crew member aboard the HMS Beluga, an old Royal Navy submarine that is now too far outclassed in combat by the modern submarines too be used by the Royal Navy so is now owned by a merchant captain. You are in the remains of what was once London but is now covered by ocean after the old great ones submerged the world in water to punish the human race for the winner-less war. However the human race was the only land dwelling species to survive and now live under the water except for the few man made islands. Most of life takes place in the bubbled cities full of air unless you are one of the lucky few with a submarine. Life is hard but humanity continues on the hope that the great artefact will be found. The great artefact left by the creature of uncomprihensible nature. The artefact that will bring back the land. The artefact that none are sure exists.