The immortal fish is an immersion issue more than anything
I find the decision to make all of the fauna completely immune to damage caused by the player takes me out of the experience a bit. I never really went out of my way to kill things in Sub1, but it made the fish feel more like actual real creatures when they died whenever I accidentally hit one with my seamoth, or made biters bleed when I hit them with the knife for not leaving me alone.
In Subnautica 2 nothing really reacts to you in that way, and the game feels "game-ier" for it. It feels like I don't have the same impact on the world as I did in the first game. I don't really want the ability to kill leviathans, I don't need a reward for smacking a nibbler to death, but it certainly made stalkers feel more real when you could hit them and they'd bleed and even die. Everything here just feels less "real".