I happen to agree that we've become too sensitive when it comes to protecting our children and I suppose in the USA it is really extreme.
But I'm fully on the govt's side in this case.
When a child rides a bicyle a few miles or wanders around by herself, it's likely she'll in be able to call for help if she runs into trouble. Or if she doesnt come home at the end of the day, it will be noticed and there's only so many places she could have gone for (unless being kidnapped).
So, if she gets in trouble over her head, there's a good chance someone somehow will be able to help in time.
Out on the ocean? Not likely. It's simply not that crowded on the open sea. Noone can help her in time if something happens and there simply are so many more things that can go wrong. Too many variables to consider.
So, as the govt has a duty to protect its citizens and esp. minors, it's the right decision to not let her go. While I'm not fond of the idea of a govt protecting people from themselves, this is a valid exception.
And anyway, I wonder how much this is really the girl's dream or her parents idea of showing her off and profiting from it.