Morgana is not going to climb up a bookshelf and wait, while the Doctor, who has wound straight through his shoulder, does something.

"Doctor," she says seriously, over the rushing water, "I can swim." She's not going to mention she is in no way dressed for it. Gowns do tend to get tangled.

"We are not separating." So, instead of climbing up the bookshelf, she follows him. "Whatever is trying to kill us wanted us in here, because there is no other way out." Of that, she is convinced.