I agree with the poster who said to remind her the night before, remind her again to check when you pick her up, and that should be the end of your responsibility. From what you've said it is ONLY the swimsuit she is forgetting - does she have everything else such as towels, shampoo, hair brush or does she scrounge those from you too? If it's just the swimsuit then it sounds deliberate.
If you have actually seen her swimsuit (the implication from your post was that she often/nearly always forgets it) you could also say that she looks so much better in her own one anyway. If she doesn't have one, ask her why she signed up for swimming without having the gear first. She wouldn't have signed up for ice hockey without skates, or tennis without a racquet would she? Although if you had spare ones I suppose she might have done :-)
Could you find a really cheapo one in a thrift store, not necessarily in the right size, and say that as your other one has gone missing, this is your spare-to-loan one now.