There was actually a study on that a while back:
http://www.breitbart.com/article.php?id=D90D12C0B&show_article=1
What they don’t address is why some people that don’t swim will act like they can anyway, and end up drowning. But that can probably be answered just by saying that stupid/careless people can be found anywhere.
People have done studies that have shown people who can’t do things or know subjects well, tend to overestimate their abilities (like the duffer on the golf course who lowers his handicap) while experts in their field tend to downplay or underestiamte what they can do.