hmm thats a bit of a tricky one, because you don't want them trying it out of curiosity, and you don't want your daughter feeling bad if they do. Also not a good idea to try and stop your daughter from swimming. I'm no expert in this, but how about something like;
"you know (daughter) has been sick for a while, well she hurt herself while she was sick, and she has some scars from it. She is ok and getting better now though, so don't worry if you see any, ok"
Something like that perhaps? and obviously make sure you answer any questions they have. And as much as it feels you should shelter and protect children, honesty is usually the best policy, as they often find out at least some of the truth for themselves