There are similarities no doubt PM. Subnautica doesn't bill itself as a horror game, though it has these elements. Soma definitely is a horror game, much more of a psychological trip than Subnautica is. Subnautica is a survival game, with the scares springing from nature, or the fact that alien creatures are inherently frightening. There is danger, in spades, but no scripted.jump scare sort of stuff.
It can also be scary since depths, darkness and sounds can all combine to put your mind in a state of apprehension and anxiety. I think the natural human base reaction to things like darkness, the unknown and unseen, as well as the fear of drowning are what makes Subnautica frightening.
Compared to Soma, Subnautica is open, brighter and much more peaceful. There are places that don't fit this description of course, but the threats are mostly avoidable if you're paying attention.
Animals flee this hell, the hardest stones cannot bear it for long. Only men endure