I've never gone wrong with a heavy duty rubber liner over a good underlay. I built a larger pond once with clay, built up with additional layer of bentonite. It worked pretty well, but I moved in the following year so don't know how it held up. I'd like to try it again some day when I have more land. I use lots of stock tanks, from 50-1000 gallons, and some of these are sunk completely in the ground. I don't care much for the preformed liners–I can't get the ground under them perfectly matched up to the shape, especially once the weight of the water settles the ground, and they are much easier to damage than thick rubber–especially if you miss a rock somewhere under them. I do use the 35 gallon round preformed ponds, meant to be freestanding above ground, as sunk mini-ponds to allow me to plant lotus and bog plants in perennial garden beds (I also use these same containers as planters to sink into the larger ponds for growing large plants like Euryale).
[Snipped some rambling. Must have been in an odd mood, and got a bit off-topic.]
