My primary job for the past 44 yrs has been to stabilize well bores. This included controlling fluid/water losses when drilling sands, fractured formations, and reducing water losses into water absorbing shales and clays. We used polymers and clays(bentonite etc) and corn starch when sealing off these formations. It's my opinion that a combination of both clay along starch works best to reduce fluid/water losses. But I have some concerns when using starch in fresh water ponds. So polymers with clays are most likely be the best way to go. There are several ways it might be applied depending on what the problem might be. I am not trying to step on Tj's recommendation's or to market another product. Fact is I have bought SoilFloc off of Tj to seal off vertical walls in my sediment pond. But here only to pass along a little experience on how to seal off water seepage. Here is a little information. Phillips Petroleum designed a product (polymer) back in the 60's that was used to spread on the ground so when it rained the water would run off and not absorbed into the ground. Or what we call water loss control. This was designed to stop the mud slides in California. This product was adapted to the drilling of wells to reduce water losses which most likely lead to water loss control in ponds.
Do not judge me by the politicians in my City, State or Federal Government.