You don't have to have Thassalophobia to be scared
You don't have to have Thassalophobia to be scared of a bottomless void where you are not the apex predator and are in fact prey.
This is not just true of Subnautica but of real life. Reaching an underwater cliff and knowing there are sharks, giant squid, massive whales that can accidentally kill you, among other smaller but no less deadly threats; is truly a scary experience.
It's not a phobia, it's recognition of danger. You aren't equiped to survive in that location, you don't have the tools to keep yourself safe, there is no shelter nearby. The crazy people are the ones who aren't scared.