I don't consider Subnautica to be horror, either, but I recently watched someone on YouTube who's deathly afraid of open ocean play it. At the beginning, she even stood on top of her floating escape capsule for a minute, working up the courage and steadying her breathing, before jumping into the water for the first time. She got more comfortable and played about 8 hours, but once she started having to go really deep, she had to quit. Ironically, she mostly plays horror games, but Subnautica was too stressful for her. So, maybe it is horror for some people. Along the same lines, I played Grounded earlier this year, and that's very much like Subnautica in gameplay, mechanics and so on. I wouldn't consider that horror, either, but if you're deathly afraid of spiders or bugs, I suppose that it could be.