Bentonite wouldn't help your scenario with a point leak since this sort of clay doesnt have the integrity to bridge an actual hole. It will only work by filling voids between soil particles. It also takes very special care to get the stuff to hold on slopes steeper than three to one.
When clays are exposed to a calcium source (gypsum or lime) the calcium can replace the sodium in the clays and cause shrinkage. This isn't going to happen overnight, however. If you are in an area of hard water, I'd give the pond a few months to see if it clears itself.
