Access to seven early development Prototype builds was sold within the earliest access campaign, selling the Subnautica Special Edition, which included other items such as the Special Hull Plate Upgrade and extra soundtracks.
Over the course of about one year Subnautica went through a number of Prototype builds. A lot of features were explored, like modular sub construction and base building. Also mechanics like dynamic lava, submarine ballast controls and more. In the first prototype of the game "Descend" had a multiplayer function, though it did not do anything.
Notable is the change within the name from Descend to Subnautica.
Prototype #7 was lost at sea, according to Obraxis.
Eventually the Subnautica Terrain Test could be seen as another Prototype.
Prototypes was originally going to be available as free DLCs for owners of Subnautica however it seem the developers have abandoned this plan.
Available prototypes (File Sizes)
Prototype Descend 1
Size: 40.03 MB
Prototype Descend 2
Size: 91.7 MB
Prototype Descend 3
Size: 63.2 MB
Prototype Subnautica 4
Size: 65.4 MB
Prototype Subnautica 5.1
Size: 67.1 MB
Prototype Subnautica 6
Size: 74.3 MB
Prototype Subnautica 8 ( 7 )
Size: 258.7 MB
Prototype Subnautica Terrain Test
Size: 200 MB