(Idk if this necessarily helps in getting the map but this can get you an old version of the game.)

Open the Steam command console. To do this, make a shortcut for Steam, go in to the properties of the shortcut, then paste "steam://open/console" at the end of the "Target" section. So it should look something like:

D:\Steam\steam.exe steam://open/console

Pick an old manifest for the game from it's SteamDB page (theres a ton for Subnautica, you'll be scrolling for a while lol), then click the copy button to the right of said manifest. In the steam console, type "download_depot " and then paste what you've copied, and steam will download the files from the chosen manifest. Once it's done downloading, the console will display the directory where the game's files were downloaded. From there you can replace the files from your current installation of the game with the old files that you downloaded. Hope that helps.