The basic rundown is this - the entire planet is infected with a disease called the Carrar (I may be spelling it wrong, I don't remember). It's infected because some super advanced aliens known only as "The Precursors" were studying it in underwater labs on the planet but due to an accident the lab was destroyed and the disease got out. The Precursors put a heavy quarantine on the planet and built that giant gun to destroy anything that gets close and/or tries to leave. This is how the Aurora got destroyed. Since you're on the planet now you're infected as well and also barred from leaving.

The rest of the game involves going deeper in to the water to find these Precursor labs and piece together a cure for the disease. You discover that the Precursors were mostly looking at an intelligent being known as a "Sea Emporer Leviathan" as a potential cure - the Emporers are immune and apparently secrete an enzyme that might treat the infection. It turns out the Emporer is still alive - she's been trapped in a Precursor base for centuries since she's too big to get out.

She's the mysterious creature you get visions of - those visions are her contacting you telepathically asking for help. You see, she has these eggs - her children. Unlike her, the children are small enough to leave and go live their lives in open water. The only problem is that the eggs need a specific enzyme in order to hatch - an enzyme that the Emporer has no means of obtaining. She asks you to craft it for her and hatch her babies.

Once you do, and once she confirms her children are healthy and free (and producing the cure enzyme all over the damn place) the Emporer is finally at peace and let's herself die :(

Now that you have the cure you're able to shut down the gun, which allows you to build your escape rocket and finally leave the planet. As you go the Emporer sends you one last telepathic message.

The Degasi was a Mongolian ship manned by a crew of six that crashed on Planet 4546B en route to a space station. The crash occurred around a decade prior to the events of Subnautica, in identical circumstances to the Aurora.

Paul Torgal mentions in one of the PDAs that the Degasi was taking a "detour", implying it was not on its normal route and was shot down.

Three crew members of the Degasi survived the crash, these three were Paul Torgal, his son Bart Torgal, and Marguerit Maida, a hired mercenary.

Paul, Bart, and Marguerit scrapped the Degasi in order to construct the seabases found on the Floating Island. They went on to build a base in the Jellyshroom Cave, and then in the Deep Grand Reef.

PDAs left by the Degasi crew can be found on the Data Downloads page.

Paul Torgal
Age at time of disappearance: 79

Gender: Male

Information: Majority shareholder in the Torgal Corporation. Beneficiary of life extension technologies. Captain of the Degasi. Father of Bart Torgal. His last PDA suggests he died after the destruction of the Deep Grand Reef base, having followed lights of unknown origin.

Bart Torgal
Age at time of disappearance: 19

Gender: Male

Information: Only child of Paul Torgal, heir to the Torgal Corporation. Beneficiary of cerebral implants. Kept the remaining Degasi crew alive after the crash by growing food. After the destruction of the Deep Grand Reef base by a Reaper Leviathan, Bart managed to survive for an unspecified amount of time, before he fled to the Floating Island base, where he made his final log in the western observatory and succumbed to Kharaa.

Marguerit Maida
Age at time of disappearance: 42

Gender: Female

Information: A mercenary hired to protect the Degasi in case of pirate attacks. Encouraged Paul and Bart to set up habitats under the surface. Very experienced, wrestled Stalkers for their teeth and injured a Reaper Leviathan to the extent that it almost died. The last mention of her stated that she was attempting to kill the Reaper that stalked her to the habitat, and was responsible for the destruction of the Deep Grand Reef Degasi Seabase.