So a thing I think is interesting is how clearly Subnautica has a early/mid/late/endgame structure, considering it's an open world game and you don't have to do the plot "in order". Discovering new tech opens up areas of the game that were strictly inaccessible before.

Early game is everything before you get the Seamoth. At the very first you're figuring out basic survival, then once that's down and you get an air tank or two you can start actually exploring. You're still tethered to the surface though, and anything below 250ish meters is basically inaccessible for all but the shortest of visits.

Getting the Seamoth opens up the midgame. Suddenly you no longer need to keep bouncing to the surface, which is huge for serious exploration. Almost every biome becomes accessible as you get the first two depth upgrades. Collecting food and water has become a routinized chore.

The mid to late game transition is a touch fuzzier, but I think it occurs when you either get the Seamoth 900m depth upgrade or build the Cyclops. Delving into Lost River and beyond involves a novel set of exploration challenges. By this point you also probably feel like you have a pretty good handle on the overall layout of the map, just need to fill in the remaining corners. Food and water are largely automated.