Bentonite will improve the quality of the soil if the clay that is there is just fair. It needs to be mixed in with the clay and the amount you use varies by the quality of the soil. It can be hit or miss, and it can be a total waste of time. It can also solve all your problems. You just don't know for sure because even if you get the right amount, mixing it with a disk and packing it down might not be right. The other issue is that it might not be your clay, it might be there is sand or gravel just under the soil that is causing a leak. There might be a spring that is draining the water out of the pond. Springs can fill a pond, and they can drain a pond when the table water is low. Then there is the quality of the dam. How was it compacted. Most just get by with running a dozer over it. But a dozer is the very worse tool for compacting soil. The tracks are designed to float over the ground and have the lowest possible weight per square foot possible. Just piling dirt up for a dam on existing dirt leaves a layer of two different types of soils for the water to pass through, so digging a core trench helps with that. The core trench also locks the dam in place so it doesn't slide away or move when the pond fills up with water pushing it out. The size and weight of the dam has to be enough to hold the water back. The core trench locks the dam in place.
Liners work, but they can be damaged pretty easily. They are a big upfront expense that seem to last a certain length of time, then fail. How much you spend usually reflects the quality of the liner. How thick it is and what it's treated with to prevent breakdown over time.
Pond Boss has lots of information, but it's hard to find anything usefull and the core group of posters over there just turn everything into a joke towards each other. Most of them hired somebody else to dig their pond, or they bought a place with an existing pond. I've given up going there for information.
Eddie