Yeah, sometimes subtle really is the best. A girl can get easily offended, if she respects herself that is, if you come on too strong and act like all you're interested in is sex. If that is your only interest, there are plenty of women in the business. You don't have to waste your time on subtleties with them.
I say, if you're looking for a meaningful relationship, take your time. And whatever you do, don't ask for her number the first time you meet her. And I think probably the worst thing you can do is give her your number. Don't do that. Trust me.
If a self respecting girl senses that all you're after is sex, she'll be able to smell it a mile away. Chances are, if she had enough determination and ability to get to the level she's at with swimming, she probably thinks highly of herself and her body. Take it slow with her if you want anything to come out of it. Try to act like you're not extremely interested. Leave her guessing.