I tried the demo and it does look nice. Not so keen on having to move your arms to swim, as that just gets tiring. (I think there's also teleport but no smooth movement).
For an underwater experience, I really love The Blu. It's so chill, but breathtakingly beautiful.
As for Subnautica, it's amazing. There is now an incredible mod which adds proper VR controls too. (I had to play on gamepad). I got all the way through to near the end of the game before I found out about it.