I agree with the leaking water calls. I usually say.. "have it done right in the FIRST place"... Thats the best way to stop a leak. I'm supposed to speak with the product reps this week and hopefully i can get some to try out. obviously they say the best method is to put it down before the pond fills and till it in, but it's supposed to be able to function as a stand-alone patch right over sand, cracks, drains, etc...
http://www.aquablokinfo.com/images/site_docs/pond%20seal%20press%20release.pdfthat's a press release I found. Seems intriguing. Mr Otto (or any soils folks), does the theory seem sound? I am what you may consider "soil-ignorant".