Subnautica is amazing. The natural unease in exploring the depths of an ocean is something I didn't realize I would love until I finally got that game a few months ago. Also constructing your base in a modular way is good fun as well.
Subnautica 1 yes. Subnautica 2 (below zero) is far less epic. The world is smaller and denser. There aren't the big open spaces, much less deep water. It's prettier and has some better features here and there, but lacks the majesty of the original.
That's unfortunate. I hadn't picked it up yet, still working through the first one.
Subnautica 2 is add-on DLC for the first game. It just doesn’t feel like a new game.
No it isn't. I own both. "Below Zero" is standalone. It uses most of the same mechanics and tools but is not an add-on.