I would've also been offended because of the way it was said and the tone used to address it. However, it is important that she mentioned it as the swim diaper is not made to hold pee and could make a pretty important mess if the child has a pee accident while wearing it. But, she should've approached it in a respectful and kind way and then you would certainly not have taken offense to it.
People tend to forget that tone and approach have a bigger impact on the way the message is received that the words themselves!
I think you would feel better talking to her about this tonight, maybe simply saying that you did not realize that the swim diaper did not absorb at all and that you grabbed it quickly but that it was a one time error. Then proceed by saying that should she ever have any worries or concerns she should feel free to discuss them with you but that you felt targeted by her approach regarding this issue!? It's hard to have these kinds of discussion but I think if we let it slip just one time we are allowing it to happen again.
Good luck