Hindsight is 20/20.
I wish I would have found a "now" friend earlier. We were both digging ponds at the same time. He has pure clay, I have almost pure sand. He needed sand, I needed clay.... The dump trucks could have gone both directions loaded. We ran into each other a year later.
Build cover BEFORE the pond started filling.
I would also have taken out the island - would have gotten more water volume and could have gone deeper.
I wish we would have finished a day or two earlier and that would have given myself time to put erosion mats all around the pond. I left for a 10 day hunt in Wyoming and the day that I left Hurricane Ike rolled through the area, dropping 12" rain in 24 hours and washing 3' of sand back into the pond.