I get freaked out in underwater segments in 3D video games. IIRC Quake had some underwater levels and they totally freaked me out. Same with Metroid Prime even though they were small. The latest as Subnautica VR. I couldn't even play period. Just too terrifying for some reason. I jumped in the water, swam down a little and then was like "Nope!".

I've thought maybe it's that things can come from below where as generally that doesn't happen on the ground. It could also be the reverb-y sounds. I have no idea. I love the ocean and swimming so ...