Deep Black takes place almost entirely underwater in the near future, in a world of unsteadiness, chaos, espionage, terrorism, and a desperate fight for world supremacy and possession of sophisticated biological weapons.
Most of the action in Deep Black takes place underwater, with players navigating the depths with "specialized underwater jet packs with integrated harpoons, mini submersibles and other high tech equipment." But it's not all swimming' and shooting' -- 505 promises a storyline with "complex science-fiction mystery, espionage, and bio-terror.
Source: Wikipedia, "Deep Black (video game)," available under the CC-BY-SA License.