NTA. I guess what some aren’t seeing here is that this isn’t about your son learning how to swim. He needs to learn how to swim but if I were a parent, I’d like to know where my child is being taken and what he or she would be doing. Swimming is a necessary skill but learning comes with risks. She needed your consent which she didn’t get.

Did the sitter bother to check if your son has a medical condition that could put him at risk simply by performing certain outdoor activities? Good he doesn’t, but what if he did?

I don’t think she’s an AH either. I see her as well intentioned but young and naive. I understand why she wanted to take him to swim lessons but she should learn her boundaries. Perhaps give her another chance but communicate what she can and can’t do, and explain why?