re: How to get a pond to retain water.....

drain the pond

dig it deeper

line the sandy layer (should stick out like a sore thumb with Bentonite, which is clay and you can get it from a drilling service company (Baker, Halliburton, Schlumberger, or any drilling fluids company.) or call a smaller drilling mud company and buy some. You can use used mud, but it is hard to haul and could be contaminated with norm or shite that could kill fish. It probably would be safe but there is a chance.

You could call a directional drilling company that drills waterlines and phone/power lines underground and thay may have a bunch od clay they would bring to you for free because it would save them a disposal fee.