The fact is myth is fact.
The story of Washington throwing a rock across the Rappahannock comes to us from Mason (a/k/a Parson) Weems, and as one historian told me, "The trouble with Weems is that he isn't lying all the time."
Indeed we did try to reproduce the feat for Rediscovering George Washington, our PBS documentary, with half a dozen pitchers from the Stafford High School baseball team. Two of them reached the other side twice. It is not superhuman, just hard, and for young George to have done it would have been impressive.