Here are some questions and ideas:
Does she have any old swimsuits not worn anymore? Then use those backed with light-weight interfacing.
Does her swim team have a logo and specific colors?
I like the idea of little figures, or dolls, or dress forms, etc.
Has she won any ribbons or medals? You could embroider this information, or put together the information and print it out on fabric with your computer and printer. This could be in a large block on the back of the quilt.
Use a pool, shape to the quilt. Adding in the pool markings and diving board, etc. Add in small pool toy items, such as beach/pool balls, flip-flops, floats, noodles, skinners, umbrellas, etc. Use swimsuit material for these items. Use terry cloth for the backing. It could look like a large beach towel.
Hope some of these ideas are helpful. Good luck with your project. Keep us updated with lots of pictures. We love to see projects from start to finish.
Pam M