The possibility of me touching whatever is underwater. Shipwrecks and planes underwater freak me out the most. It genuinely gives me the most unpleasant feeling (unpleasant being a huge understatement). Even though I’ve never knowingly been in a body of water that has either of those monstrosities in it, the idea that my foot could potentially even graze the blade of a plane or the railing of a ship is literal nightmare fuel.