One other thought which has escaped this thread...if you have topsoil sitting on top of clay, you probably don't have an issue. Here's where you would have an issue. Water seeks the path of least resistance. As long has you have clay contacting clay around the entire pond, (whether the clay is two feet or nine feet down,with sandy soil on top), you will have a seal. If the 'hole' in a shallow area leaks, water must pass through it via a connecting area of porous soil or other fracture, then pass beyond or around the dam to escape.
While bentonite or clay does no harm, it may be money unnecessarily spent.
My leaning would be to buy a few bags of bentonite, dig out the questionable area, work the bentonite into the sandy spot with a rake, mix with existing soils, and forget about it.
