geotextile is what i used, i have been very happy with it and it's great if you ever need to move a pond, after two years under a liner it was still in the same condition it was when put under the liner. But for cheaper substitute, a tarp, old carpet, Weed barrier, and newspaper have all been used sucsessfully.
landscape fabric heavy duty I used .It was 29 dollars for a 6 ft. wide roll by 200 ft. long at Ollies discount store .I had to redo my pond this summer after a dog got in my pond and ruined the liner .I still left the old liner material in ,I just took out the ruined pond liner because I did not want water to get trapped between the 2 liners .Lowes and Home Depot both sell the underliner and Some places ship free if you buy so much from them.Some also use old carpet or carpet padding .
Good choice, for the money it's a very good product, you may have spent a little more for it but it does give a really good protection and it's very light weight and easy to work with. LOT stonger than it will appear. I was very impressed when I had to remove it from my friends pond at just how well it had help up and how good it did over that period of time. BTW, he had rocks and roots to deal with when he dug, and no problems from either using the underlayment.
My ground is a clay sand mix. On my most recent pond attempt I tried something new. I soaked the hole to turn the first few inches of the bottom into soft clay. The idea is that any lumps, digging produced many rock hard ones, that might happen to find their way into the hole during liner installation will get pushed down into the wet clay, soften and be no more.
I expect over time the moisture will diffuse into the surrounding soil and create a smooth firm bottom. The plan is to rip this pond out in a year or two and I can give a post mortem on the process.
This only works if you have rock free ground. Or maybe it will work so long as the pond bottom is below the frost line and you make sure there are no rock is the soft clay.
I used old carpet with shredded newspaper and regular paper on top of the carpet. The shredded paper binds together and forms a powder like substance like baking flour. If water gets to it via a leak if just beads on the paper like a drop of water will do in flour.
I used some Home Depot “slumlord special” carpet that was leftover from a project, it is tough as heck and only about $0.50/sqft. Extra points if you can repurpose some old carpet that would have otherwise gone to the landfill.