A qualification for my advice: I have not applied to boarding schools before, so this advice is based on things I have read here at CC.
At this point, AOs will have tons of emails to get through. If it’s already in your application, they will see it. Sending an email might irritate your AO.
If you have something for swimming, you might want to look into contacting a coach. I know nothing about athletics, so it would be best to get someone else’s advice here.