Subnautica presents some interesting challenges for audio, a big one is finding a way to record high quality audio underwater. this requires some specialized gear. the old school way of doing this is to put your expensive mic in a condom, hope it's water tight and dunk it underwater ( 100% seems like a bad idea to me). So I got a dedicated hydrophone: 

In this video I'm also using a GoPro to capture sounds on actual scuba dives and to document the recording process. Unfortunately the GoPro's audio quality is too poor for sound design but the videos it makes and the dives themselves give me a great auditory and experiential reference i can call on for creating the soundscape for Subnautica. I found it interesting how different the underwater environment actually sounds compared to how I and probably most people imagine it does. There is so much clicking happening under the waves - is this fish chewing on coral? - I don't know.