I'm now 20 hours in so I think I can make a good (temporary) conclusion on the game.
Subnautica is basically a bit like Minecraft, except, you know, underwater. There's a lot to do and I still haven't even scratched the surface of the game yet. Crafting, exploring, building and upgrading your seabase. There's a wide variety of plant life and sea creatures, some friendly and will run away (damn you, peepers >.>), some aggressive, like the reaper leviathan and Bone Sharks.
The location variety is really good, too, and the game is incredibly beautiful.
Now, would I recommend it? Well, that depends. I have a few gripes with the game, mostly technical. I have a pretty beefy PC, but I see frame drops every now and then. I know that the game is still in early access, so I imagine it will become much more optimized as the game progresses. Right now, I wouldn't recommend it if you have somewhat crummy specs.
However, if you have a good PC, and you like survival games, I absolutely would recommend this game!
My advice, get it now instead of later if you're on the fence and have a good PC. It may well go up to double the price when the game is fully released.
It's definitey miles better than most early access games, and the devs actually want feedback and to make the game better, unlike some other developers that might scoff at advice and release a bug filled, poorly optimized game.
I may edit this review as the game goes along.