As of this post I have about 5 hours on Subnautica. Around 2 hours in Survival and 3 hours in Creative. If you want to check it out for yourself the game it is free till the end of the month on the Epic Games Launcher, the same place you go to play Fortnite on PC.
First off Subnautica is a survival game where you are the sole survivor of a spaceship crash onto a planet that is entirely water. When starting off it was pretty straightforward, it provided you with some food and water and your escape pod for shelter. The beginning was pretty slow because you have just get some basic materials. But, after the slow start the game really picks up there are so many biomes, shipwrecks, destroyed escape pods to explore and from these new areas your character learns how to create new vehicles and how to make bases and so many really cool things. After playing a little bit in Survival I decided to jump into Creative to see as much of the game as I could. I proceeded to create the vehicles and tried making a base of my own and it really surpassed my expectations. The vehicles I played with and thought were really fun was the robotic suit that lets you go around in the depths while having a sense of security, the little underwater submarine which made going around to gather materials millions of times easier, and my personal favorite the full fledged submarine which allows you to go to depths that I didn’t even know existed and the whole submarine is accessible and allows storage for the robot suit or mini-sub and when using it felt like a mobile home.