It's not really about water skiing. It's more about not water skiing.
(My kids have heard this one so many times they know it by heart)
The summer I turned seventeen, I won a week's stay at the most terrific summer camp in the known world. Unlike Camp Columbia (http://www.campcolumbia.com/index1.php), where I had been for a couple of weeks in other summers, Camp Owaissi (http://www.campoac.com/) had all the bells and whistles, including boys. I was taking classes during the day, in a Youth Leadership program (don't ask), and there were all sorts of activities for my free time. Kids were supervised very subtly. There were hikes and swimming and board games and dances and bonfires (with singing) and all manner of water sports.
Except for personal water craft. They hadn't been invented yet. We had canoes.
Which is where the twins came in. I couldn't tell them apart, but they looked sort of like those triplets in "Slap Shot." They took me for a canoe ride, and were so thrilled to be out there, paddling away, that the one in front rose up on his knees, and twisted around to tell his brother something. So, yeah...the canoe capsized and we were in the lake.
The next day, the two of them asked me if I would like to go water skiing.
They had dumped me in the water from a canoe! There was no way I was going to trust them with a motor boat! I figured that I would learn water skiing at a later date.
By the time I got to be thirty, and no one else had offered to teach me how to water ski, the thought entered my thick head that maybe it wasn't going to happen.
Sure enough, to this day, I don't know how to water ski. I'm not so thrilled with the idea now, to be honest, but back in the day, I might have enjoyed it. Back when I had arm muscles...
So the reason my kids have heard it so much is because it's such a good illustration of the idea that an opportunity that knocks deserves to be answered. It may never knock again.