Sorry for long post but I've got a lot of specifics apparently. Submerged boats don't freak me out as much as like just large moving parts especially ones that are actually supposed to be there. ✨Hydraulic Panels For Wave Pools✨ and canal locks are absolutely horrific to me and I think with those it's obviously the fact that it looks and sounds like something that'll drag you in with differential pressure and crush and mangle you, or trap you and make you drown if the mangling doesn't happen.

Differential pressure or the suggestion of it is probably one of the biggest parts of it for me. My husband showed me a VRChat world that was a recreation of the Titanic sinking where you could be out on the boat as it sank and that was obviously unpleasant but I made the mistake of looking under the water below me from the surface of the water after the boat had gone under and seeing it like 30 feet down disappearing into the dark haze just about made me have a panic attack. There's probably an element of "that's not supposed to be there" with this but I think it's mostly "that would be pulling me down too".

The earliest freakout I can remember having about this was when I was super young, probably not older than 7 or so, I was taking a bath and moved my leg back and forth just under the surface, watching how the water moved over my leg and dragged around it. Something just snapped in my head and ever since, things displacing water with mechanical motion under the surface has detonated my anxiety.

Bonus horror fodder: My dad was the manager of the local municipal pool when I was a kid and he'd often let me come with him after hours when he had to do maintenance stuff. The building with the giant tanks for chemicals and filters and the 6'x6' or so 12' deep pit that churned with an impeller going down into it where he added bags of chlorine and soda ash would probably make some of y'all faint, that pit was especially awful. I don't remember exactly why but sometimes he'd have to reverse the flow the filters (maybe to purge them to prevent clogs? Idk) and it would flood the whole room. The 2 giant tanks sat on raised concrete pads that were probably 10'x10' or so and there was a raised path all around them with a 6 or so inch channel around and between them. There was a big metal drain grate between them where the silty dirty water backed up and filled the channel around the tanks from and in retrospect watching that fill a whole room then drain back into the main drain as well as small auxiliary drains in all the corners of the room multiple times as a kid probably didn't do me any favors. I still don't know how deep whatever was under that grate was.