Its creator Sunrunner says it's still in "very early stages of development", but the foundation for a full co-op playthrough is there: it has synced player movement, basic animations, base building and item dropping, as well as a rudimentary chat system.
Because it's so early, Sunrunner isn't recommended for "casual play", but if you don't want to wait for extra polish you can download the installer from its Nexus page (you'll have to login to your Nexus account first).
Clearly, players aren't too bothered that it still has a long way to go, and it's been downloaded more than 35,000 times since it was added to Nexus Mods last week.
It's open-source, so the hope is that other modders will be able to iterate on it in future. The Unknown Worlds Entertainment dev team even commented on the mod last year, saying they "fully support" the project but that it would be "very hard" to get a multiplayer Subnautica mod looking as polished as the singleplayer game.
You can watch some footage from the mod below.
Update: I originally wrote that you have to manually install the mod via instructions on its wiki page—you can actually grab an installer file from the mod's Nexus page (it's in the top-right).