Hi Master of None,
If you are interested in how to best develop your DD's swimming talent this book by Dr. Alan Arata "Parent's Guide to Swimming" may be of interest to you - it has helped me a lot.

http://www.amazon.com/PARENTS-GUIDE-SWIMMING-Alan-Arata/dp/1410725391

Age 8 is still very young for swimming and there are many years ahead (as Dr. Arata will also point out) - so truly developing their love for swimming for the long-term is the key at this age.

If the family issue is more on how to develop one of the many talents that your DD has so she learns life lessons in the process (that is what it is about, after all), then from your description of your DD at this time, swimming may or may not necessarily be the right activity to teach such lessons. Hard to say from just reading, but, surely, at 8, it does not hurt to take a break and come back few months later and that does not brand them as a quitter, since at the earlier ages, it's good to explore their different potential passions anyways, and there is the limitations of their young, small bodies, energy, time etc etc.

You will be amazed that even a year or two later, your DD will likely show more initiation and focus. So many factors can impact how an 8-yr old thinks and view activities. By 10 or 11, kids start to better develop their sense of self and what they want to do. (I caveat by saying, in the end, it depends on the child and their development).

I think it is good that you want to teach her the lessons of perseverance, consistent hard work etc. Best of luck! I"m sure you will get it right.

BTW: I do agree having child good at everything can be a challenge - the challenge becomes, what do you start to limit or de-prioritize, especially when everything that they do becomes higher level and more demanding, yet what do they give up? We face that right now, with my DD9 - I feel that there is not enough time in the day or week (in her case, it is year-around swimming, competitive tennis, violin performances and academics.) Hard to figure out as a parent, what is the right approach to support and/or manage all these, especially if you have other children and their separate interests.