Quote Originally Posted by vivi_ultima
3) You can. Just mash the X (English) or O (Japanese) repeatedly, you'll get it eventually.
An addendum; the Onion Knight is hidden in one of the far bottom corners, closer to the door. When the camera angle changes to be a side view of Tidus swiming, continue into the crack, and mash the Confirm button to access it. Obviously, I refer to where you fight Geosagaeno.