> Something about journeying to the deep ocean seems so much more terrifying than space.

> At least you can see past the end of your nose in space. In the deep ocean it’s truly pitch black.

Yup, I think it's the darkness and short distance visibility that makes for thalassophobia in most cases, at least it is for me. It scares me when I cannot see the ground, even when I know that it's only a few meters deep. However, I can stand on the top of a building and look down without any fear. BTW, I recommend r/thalassophobia.