I don't have much experience with leaking ponds, but I have used quite a bit of bentonite. The way I have used it is by pounding holes into the soft poorly compacted clay in my dam and pounding dry bentonite grains into the hole till it is stuffed and won't take any more, or by making a bentonite soup mixture and pouring this into the cracks in a dry clay dam in the middle of summer.
Both of these methods seem to have worked for me.
But having now mixed over 150 lb of bentonite into slurries for pouring into a dry dam, and having a lot of very hands on experience as to what betonite does in water, I really doubt bentonite would be able to stop even a minor leak if it is put into the water. Without some sort of containment, bentonite in contact with water doesn't seem to have any cohesion that would keep it together to resist even a small current. Without being constrained by surround soil, where ever any water was moving or even seeping, I think it would probably wash away in a short amount of time. Once bentonite is in water it becomes pretty close to liquid itself within 24 hours.
Thats probably why people don't seem to have a lot of success just dropping it in the water.
