Bentonite has been used for years as a sealant in farm ponds, fishery ponds and other larger earth-bottom bodies of water. It DOES NOT completely waterproof, but rather reduces seepage to a very minute level. To be effective, it must be applied correctly, which involves tilling and compacting. The additional costs in labor and machinery use would far exceed the cost of liner in a pondless project.
"Everybody is ignorant, only on different subjects." - Will Rogers