The obvious part I liked best was that Helen Bowers included swimming as part of Portman's artistic and physical development. From Elle:
Did you include any cross-training in your sessions with Portman?
I did. Ballet can be hard on your joints and when you’re on a movie set doing take after take it can become really repetitive. To give her joints a break, we’d go swimming because it allowed us to work on her endurance and elongate her muscles with zero impact. ...[Link]
Read it to see how far she swam, how often, and what strokes.