Every child is different, but I would encourage anyone who has a child interested in athletics to give them the opportunity to play. There are so many posts on this board about teaching children to work hard, athletics will do that. It will teach them skills that academics will not.
I agree with everything sallymom's said about sports but also I'm not sure how grade skipping takes away from what sports teach re hard work/etc? The OP's child will still be swimming (at least for the near future, and as long as she's interested in it).
I suppose one flip side to sallysmom's advice is that it's also easy for a kid who is uber-focused on a competitive sport (this is taken straight from my competitive and talented athletic dd, not a hypothetical situation)... can be so focused on that sport that they aren't prioritizing school above sport, and hence they are very happy to skate by with easy academics so they can spend all their free hours working at their sport. I suspect my dd would not ask to grade skip if she thought there was any chance it would mean more homework - she already would like to homeschool to give herself more time for her sport. I would not, as a parent, let her choose an academic path that was too easy for her just to let her focus on her sport - I believe she needs to work at both.
polarbear
