Big Buck-

I build garden ponds up in the mountains of North Carolina. I use a geotextile underlayment prior to my liner.

I have used in the past such items as newspaper (pray for no wind and you get caught up reading the articles) old carpet, hay and sand. The two latter do not cover the sides or verticle drops enough and I had leaks over time.

To reduce the time invested in locating applying and lack of coverage of the other products the underliner is the best for your buck.

I can send you a sample if you would like to see some.