Just in case no one knows this, you can absolutely play multiplayer in Subnautica 1. Takes a tiny bit of work and a mod, but doable.
Of course it's not official, and may be bugs, but doable.
I'm curious what is going to be new in 2, thought the water genre had maxed out before doing deep and making it a horror game. Feel like anything else would just be DLC or a mod.