I agree with singapore, our DS did no sports at school, but he was a Jr Olympian and State champ for TKD. I know from experience how much goes into this, since it is a yr round sport if you compete on State/National level.
Also, I am not so sure extra-curricular in this case refers to at school, such as, NHS, it could also mean things you do on top of school. Do they have a special section for community service? If not this might be the case that they want everything you do outside of 8-3 school hours...job, volunteering, BSA, and then that is where you put TKD.
Since TKD is something that takes yrs to accomplish, if you are a black belt mention what degree. If you assist in teaching mention that. If you have won national and state championship in form, or sparring or both, mention it.
You may also want to somehow place TKD in your essay...for example, I have learned as a black belt in TKD, that discipline and respect are core aspects that I live my life by and feel that would make me an ideal candidate for USXX....or something along those lines. Don't throw it into brag, add it where it is appropriate to show something of you and that you have an outside life.
Many of the MOCS are also looking for the WCS, they don't want to put up someone that is the class valedictorian, but has nothing outside of the classroom because they know that the ECs count for appointments. ALso, at least for our MOCs did not ask for a copy of the CFA, thus they need to know you are athletic in some shape or form. Many of these committees will have an ALO, BGO, MALO as part of it, they know that the CFA will be assessed. No sports, and they get worried. You are not the first with a unique sport, many girls who apply have spent yrs outside of school in dance or cheer. Our last high school did not have a swim team, but a child last yr was recruited by the AFA for swim since he was a state champ. It happens.
Finally, swimming and diving at our school does not start until Nov, by then it is probably too late for your MOC app.