Word from a trusted source close to the production who would know is:
the part where you can see Natalie’s face (the banner pic) is all
Portman, they only used the stunt double for the part where she dives
into the water (here),
because pshaw, son, an Oscar-winning actress ain’t gonna dive into no
cold-ass Irish water. So there you have it. Aren’t you glad we
cleared that up? I know Natalie Portman’s butt has been keeping you up
at night. And now, probably.
Gentlemen, you may now masturbate in full confidence that you are not jerking it to a pale imitation. So to speak.