I wouldn't have known about the underwater double jump if not for the comment on the main itch page. Otherwise awesome! As others have pointed out this feel similar to Inside or Limbo. I think the double jump could easily be fixed by having the ball light up (orange, let's say) and make a sound when you can jump. That way the mechanic is introduced on it's own. Also make it less punishing as there was no skill involved for me, just buttonmashing, hoping it would work.
The atmosphere really sells it. I love it