I saw a theory clip of what may happen with Arya and it makes sense:
Arya had a sack of pigs blood under her shirt. She went out into town not being discrete at all to lure out the waif, waited for the waif to think she killed Arya, then climbs out of the water. Note how the actors used pigs blood in the plays, and Arya has some kind of infatuation with the actress.
Seems legit. I fucking gasped when the waif started gouging her stomach though. Thought she was finished for sure.