Gameplay
Subnautica puts you alone in a seemingly endless ocean with the sole goal of surviving. At first all you can do is grab resources and swim, but as you progress you unlock various tools and vehicles to aid you in your survival.
Story
You play as Riley, a janitor aboard the Aurora. The Aurora was tasked with building a phasegate to expand the controlled space of Alterra, the corporation funding this mission. However, upon trying to land at Planet 4546B, the Aurora gets shot down by a planetary defense system, leading to the start of the game. As you explore the planet, you are able to learn what happened to the other crew members, as well as a previous group of survivors. And eventually, you end up learning why the planet had a defense system and disable it to escape.
Graphics
Subnautica has incredibly beautiful graphics. They're able to make you "wow" while exploring the world, and they add to the fear factor perfectly. While swimming you can only see so far ahead of you, and this effect gets stronger the deeper you go, greatly empowering that fear of what could be lurking out there.
Soundtrack
The soundtrack is pretty decent, it works perfectly with the game. It works well in conjunction with the graphics, as you'll sometimes hear creatures you cannot see, making that fear all the more real.
Difficulty
Most of Subnautica's difficulty stems from a lack of knowledge, so as you learn more the game becomes easier. You learn the behavior, territory, and danger level of the creatures and how you can avoid becoming their lunch through observation.
Pricing
With it's full price being $30, and it frequently going on sale for as low as $10, it's definitely a steal. It's gameplay never gets old, so it's easy to play for hours on end without getting bored.