Does this logic ultimately end at the suicide question? Does the individual control their own life? Does this girl (or by extension her parents) control her own destiny? Is it the right of the Government to determine if the risk of harm coming to the girl is sufficient to stop her sailing quest?
It seems to me you can't ASSUME she will be in harms way by sailing singlehanded around the world. Especially if she is within a flotilla the whole way.
We can't assume that she will be in harms way, but we know for sure that her behaviour is not conductive to better survival rates if practiced by people in general. We know for sure that it is a bad precedent.
Evolutionary forces tend to eliminate from the gene pool those who tend to expose themselves to high risks at young age. This means nature will take care of things if she is allowed to sail.
I tend to believe that it makes sense to restrain her instead of expose her to evolutionary elimination of her genes/conduct (meaning herself or most of those who follow her behavior pattern). It is faster and less painful. Naturally, I will change my mind if convinced that her behaviour is conductive to better survival rates.
I do, however, agree that governments in general are poorly prepared to handle this type of situation and that their decisions are deviated by politics, with a history of failure when it comes to decisions conductive to better survival rates.