Other than in your "reply #1" (and the thread description) I don't find any published report of "20 FEET" on the internet.
Meanwhile, whether or not that figure is accurate in THIS case, I agree it could be in others. I had a terrifying experience similar to yours while I was wading along the shore of the ocean with my three year old nephew about 15ft from me.
I turned my head for no more than 5 seconds, and when I looked back, I couldn't see him. INSTANT PANIC. Luckily within the next 5 seconds, as I screamed his name and desperately scanned 360 degrees, I found he was actually in the same spot I had seen him just a moment before. But he had bent at the waist so he was less visible, and also the sun glaring in the water had made it difficult to see.
Lesson learned: NO, it's not okay to look away for even 5 seconds in that kind of situation.