I'm getting serious Endless Ocean (opens in new tab) vibes from World of Diving, a multiplayer underwater exploration game now in Steam Early Aquatic Access (opens in new tab) for your consideration. It's a game about swimming around on the sea bed, discovering sunken treasure, and taking photos of all the weird and interesting stuff in the ocean. It's not a game about killing things, but I suppose there's still time for developers Vertigo Games to add killstreaks and gory, slow-motion takedown moves.
The Early Access alpha release contains a mission editor, along with a diving base and a handful of levels themed, among other things, around sunken ships and airplanes like the Lockheed Electra. There's Oculus Rift support too, which seems like a great fit for such an exploration-focused game. The next update will see German WW2 Battleship the Bismarck added to World of Diving, while the eventual full release will boast animal companions and more fully featured multiplayer.
Here's the Early Access trailer: