Subnautica tells you almost nothing, on purpose. This is the route through it that keeps the discovery intact while stopping you from wandering into the Dunes at hour three and quitting.
No story spoilers below, only the order things become possible.
The Safe Shallows
Your lifepod lands here and everything nearby is harmless. Collect acid mushrooms, salt, quartz and copper. Craft a scanner, a repair tool, a knife, and a tank.
Scan everything. The scanner is your progression system and unscanned fragments are locked recipes.
Repair the lifepod radio and listen to the messages. They give you coordinates, which is the only navigation the game offers.
Kelp Forest and the Seaglide
Creepvine gives silicone, which gives fins and the Seaglide. The Seaglide roughly doubles your effective range and carries a light, which matters more than it sounds.
Build the Seamoth as soon as you have the fragments. Until you have it, the game is much slower than it should be.
Grassy Plateaus and your first base
Shale outcrops here hold the diamonds and gold you will need constantly.
Build a small base with a fabricator, lockers and a water filtration machine. Solar panels work fine at this depth. This is your staging point, and it should be somewhere central rather than pretty.
Jellyshroom Cave and the Moonpool
Around 200 metres. The Moonpool lets you dock and upgrade the Seamoth, and the depth modules unlock everything deeper.
The Cyclops fragments are scattered across several biomes at this depth. It is a large investment and it is the vehicle that carries the rest of the game.
Deeper: Lost River and beyond
Below 500 metres you need the Prawn Suit and upgraded depth modules on everything. The Lost River is the gateway to the endgame and it is where the story resolves.
Bring a Cyclops with fire suppression and shield upgrades. Not because of what lives there, but because the trip back is long.
What to avoid until you are ready
The Dunes, the Mountains, the Crash Zone and the Grand Reef all contain Leviathans that will destroy an early Seamoth.
If you hear a roar, leave. The game gives you plenty of warning and the deaths that frustrate people are almost always deaths where the warning was ignored.
Beacons are the map
There is no map in Subnautica and no waypoint system beyond what you place yourself.
Craft beacons early and drop them on anything you will want to find again: your base, a wreck you could not finish, a thermal vent, the entrance to a cave system. Rename them as you place them.
Players who skip beacons spend most of their playthrough lost, and being lost at 400 metres with a failing oxygen supply is how saves get abandoned.
Managing oxygen, which is the real clock
Every trip is bounded by air. Tanks help, the Seaglide helps by making trips faster, and vehicles remove the problem entirely within their depth range.
The habit worth building early: turn back at half. If a cave has consumed half your oxygen, it has consumed half your return trip too. This sounds obvious and is the single most common cause of death in the game.
Air bladders and pipes let you extend a dive cheaply. Both are worth crafting before you go deep.
The Subnautica builds guide covers blueprint priorities and depth modules.
