I suppose bentonite may work directed into the hole, but I have heard very few success stories.
Bentonite is a rock type composed of a group of swelling clays, and a widely available industrial product of the same name. As a dry clay powder, it will swell substantially when saturated with water. I think it can only work where it can be emplaced in a less than fully-hydrated state and then allowed to expand further as it takes up its maximum water content.
I do NOT believe it would work on any macro leaks. If you cemented off the big hole, and then used some bentonite to fill the gaps in the tiny spaces where the concrete did not bond to the pond bottom substrate or where the cement contracted and left tiny gaps, then it might work in that scenario.
