Subnautica is a survival game that is focused on the player trying to survive on an alien world almost entirely covered by ocean. You should build a base and find a way to go back home.
1. Fix your emergency pod
Your emergency pod is where you start the game and it is going to be your base of operations for a while. You should mend whatever is broken there and make it functional again. It will provide you with an energy-free crafting station, free medkit generator and a respawn point. This is where you start.
2. Get some food and clean water
You need to sustain yourself, so for this, you must catch and cook fish. Some of them are capable of generating clean water, so you can use them instead of bottled water. You also can use salt to make your food store for a longer period of time and also you can make a clean water generator that will turn salty ocean water into drinkable water and salt.
3. Make a base and some vehicles
You need to create a base for yourself and then you will need to make a hovercraft and a submarine. The hovercraft will allow you to explore the depths of the ocean. There you will be able to grow your food and customize your vehicles.
4. Check the main ship
Visit the main spaceship, there you will find blueprints for the rocket to go back home and some other useful resources.
5. Collect resources for the rocket
The bottom of the sea has a lot of valuable resources to help you build your rocket. You just need to find them and collect them. The resources can be used in building tools and weapons. After that, you will be ready to build the rocket and leave the planet.
Conclusion
The player starts the game by landing in their emergency pod. The player then has to find food and water to sustain themselves, fix their emergency pod, make a base, make a hovercraft and a submarine, collect resources for their rocket, and finally leave the planet.