She'll tell people later that she and her parents were boating. This is not untrue. She'll tell them other true things, too, like that she was the only one wearing a life jacket at all, and that she took it off eventually because it was hot and her parents weren't wearing them anyway. She'll tell them that they were so far out that she could barely see the shore. She'll change her mind about that one when people express skepticism that she could have swam back that far.

She won't tell them that she spent most of the process thinking about what she'd tell people afterwards; that's not a cool or even reasonable thing to be thinking about when you're drowning. And it's not like she spends the whole time thinking about it. Eventually she gets to the point where she's trying her best to swim back, but she's tired and she's never been a good swimmer anyway, and there are huge waves and there's water in her mouth and she can't breathe. 

At that point she thinks about how there probably isn't going to be an afterwards.