Spoiler warning: the following contains info related to the degasi story.
What if the Degasi landed in the sparse reef? There is evidence to prove this idea. There is one large wreck in the sparse reef, that is rather cube-like. In the data downloads, the Degasi survivors say that they stripped their ship down for resources. The very cube-like structure of the large wreck, it is possible that this is part of the Degasi, and was cut at for resources. The colors on the wreck match updd as well (wreck and degasi pictured at bottom). The Degasi had many orange lines, and on the rear section, grey sections near these lines. This wreck could be part of the rear section. Furthermore, the sparse reef has, for a reason the PDA cannot determine, very little fauna/flora diversity, and overall less fauna/flora than many other biomes. This was most likely caused by the Degasi crash landing. You may say, however, that the sparse reef is much smaller than the crash zone. The Degasi was smaller than the aurora, and crashed roughly a decade before the events of the game, along with the Degasi survivors removing all important parts of the ship(probably including the radiating core), the edges of the biomes have begun to grow back. The sparse reef’s ecosystem was probably created by the lasting radiation and pressure waves making it only suitable to a few plants(like eyestalks) for a while(though long enough for a main ecosystem to develop). Although the floating island cannot be seen from the sparse reef, the existence of skyrays allowed the survivors to deduce the presence of land and follow a skyray to it. The wreck in the reef was probably left due to running out of food supplies and electricity. As for oxygen, the Degasi probably had some sort of transport, as they wouldn’t have gotten to the grand reef or even the jellyshroom caves without one, and they had a fully equipped ship, not just a life pod(like the aurora survivors) to work with. They could probably surface every now and then for oxygen, and/or recharge whatever vehicle they had(most likely a prawn suit, due to fragments around the wreck) from the Degasi’s power(there are sparks in the wreck) when need be. Although this story/theory is most likely not intended by the developers, it is an interesting one to think of nonetheless.
Degasi Final Artwork:
Sparse Reef Wreck: