Subnautica (Legacy)
Contributed by @Greelan
Link: Save 67% on Subnautica on Steam
Descend into the depths of an alien underwater world filled with wonder and peril. Craft equipment, pilot submarines and out-smart wildlife to explore lush coral reefs, volcanoes, cave systems, and more - all while trying to survive.
The legacy beta version of the game, as well as the Nitrox mod, must also be used client-side to enable multiplayer functionality.