Deep Black is a third-person shooter where most of the action takes place underwater; the "Reloaded" part means "contains PC-exclusive features" namely Nvidia 3DVision and Razer Hydra support.
Besides regular weapons and an auto-cover system which work similarly in and out of the water, your character can use the submarine equivalent of a jetpack to "sprint" and fight currents, and a harpoon to activate switches or perform silent kills. Some close encounters are settled through button-mashing.