Subnautica is, apart from being a pretty brilliant survival game, is completely free to play on the Epic store, but it's never had a multiplayer dimension, until now, with a new mod named Nitrox aiming to bring multiplayer co-op to the game.
The mod isn't completed yet, but according to modder Sunrunner, the basics are in place and working, with the important factors such as synced player movement, animations, base building and item drops all working.
That being said, you can download an early version of the mod, and give the experience a try here. The author has open sourced the project, so it's free to download and add to, so if you're someone with some development skills, then you can get involved and contribute, too.
So if you're interested, you can download the current in-progress installer here.