I disagree that Subnautica is a sandbox game. While you can definitely just exist in the world if you want, there isn't that much to do besides progressing the game. You can build a big base if you really want or collect all of the fishes, but that's hardly enough to count as a sandbox in my opinion,
Your disagreement is incorrect.

The definition of a game sandbox is: "provides players with a high degree of creative freedom and agency to explore, interact with, and modify a game world without being restricted by a linear storyline or specific, mandatory goals."
There are certainly better sandboxes, but that doesn't stop Below Zero from being one.
Some random things that I and other people do:
Build a transit network that includes things like garages with unending energy that have recharging stations, docking stations, and food and water production. Kind of like truck stops. "Roads" to all important resources are created with beacons.
These roads can lead to networks of scanning stations and mining outposts, your bases, etc.
Create a transportation center that houses every possible variation of the sea truck.
Mega bases and cities, both floating and underwater and in some cases on land. These can actually be made useful by creating farming centers, creature breeding centers, "public" aquariums-zoos. You can make a water purification plant, etc.
IMO, the alien containment is really the most fun thing in the game.
Create the underwater CIA with spy penglings.
Collect all music discs and open a disco.
Undertake expeditions into the Void, and take on the challenge of creating a Void base.
Kill the leviathans.
etc. Plus there are many mods.