Although we dug the pond some time before we moved house, it refused to fill up. I read that wallowing pigs do a sterling job of sealing the bottoms of ponds, but I don't have any pigs so I tried stomping about in the little puddle that formed in the bottom myself, poking with a stick to simulate busy little trotters. To no avail, alas. Although the trottered places held water longer than parts that didn't get the treatment, the essential ingredient 'pig wax' was obviously missing.
So at the suggestion of our dermatologist friend, we tried the trusty 'blue sheet' (if blue sheets were removed from Japan, the society would collapse within a week). We had a big ole blue sheet with some holes in it which the young master was very keen to tape up with trusty gum tape.
The effect is not lovely, and the blue sheet may need to be hidden by some means, but we should probably wait until the pond fills (or not) before we attempt cosmetic measures.