Hestie was finding it harder to breathe naturally. It should have been an autonomous action, but somehow the idea that she was underwater had become lodged in her mind and she had to deliberately draw and release each breath or else her subconscious would attempt to hold it instead.
This feeling is so scary. I’ve felt this before when I’ve been in my head, but I can’t imagine adding the feeling of being stuck 300ft under the ocean in a ship wreck on top of that. The idea makes me feel short of breath just thinking about it.