I need to give this another shot - I just couldn't "get" this game when I first bought it, maybe it has changed enough for me to try again. (Same with NMS, I keep meaning to have another look at that too.)
Did you buy it during the early access or after launch?
It changed a lot.
I think that to "get" Subnautica, being familiar with Elite really helps, because it's the same principle. You are improving your character, your base and your vehicles which allows to explore deeper and deeper so it creates a "natural progression", but what it really is, is an "aimless diving simulator"

Everything there is, in the game, I discovered by accident and I finished the story that way. You can follow some clues, of course and the story is actually interesting (unlike Elite) but it's just a backdrop for happy and aimless diving.
I've seen some people reviewing it as a "survival horror game" but I think they didn't get it either. Yes, there are big things that will kill you, down there, but that actually never bothered me. I was never "scared" to go anywhere and big things are easy to spot and avoid. It's like saying Elite is a horror game because of Thargoids.
So that's my playstyle and advice. Go into it with a clear head and just enjoy the scenery. The rest will take care of itself.
Oh, I also like building ridiculously complex bases in ridiculous places. I think a good half of my time and material resources goes into those. Who needs a five-story aquarium? Well, I do.
