I had similar surgery at age 13 and further heart surgery about 21 months ago so I have plenty of war wounds!
First remind your daughter how incredibly brave she was to face the op in the first place and she got through it and is healthier now than before. I'm sure you do this anyway but show her all these replies so she knows it's not only her Mum who thinks so :0) Then tell her that her scar is actually a badge of honour which testifies to her bravery. I always thought of my scars
that way - even at 13.
Other people are interested in dramatic scars and why not? They are way beyond the experience of most people. Try to get her to see it as something "special" - which it IS! People will take one look and then that's it - interest over. She should absolutely continue to swim, wear any clothes she likes and pretty much ignore her scar.
She could try Bio-Oil but baby oil will do just the same things and it's cheaper :0) Treat the scar like any other piece of skin with moisturiser etc.
But more important than any lotions or potions is her attitude to it.
I hope you are reading all these answers with your Mum honey - you are a brave person. I know exactly what you are going through but I promise that it will be just fine. And maybe people are just looking at you because you are gorgeous - did you ever think of that?
Good luck - forget about the scar and enjoy your precious teenage years in the very best of health :0)