I don't have Subnautica but it is on my wishlist because you can play in VR, which is what I mostly play these days. PCVR is not as reliable on Linux as standard games, but nevertheless more than 50% of titles do work flawlessly now. Subnautica is definitely one of them - you should check for other people who've got your problem on ProtonDB. If you actually care, look into it more, you should be able to get all of those games running.