Having built several ponds the oldest one is 6 yrs old. non with leaks here's my $.02.
After excavating the pond basin, I put 4-6" of sand down to protect from any stones working their way up thru liner. Next, I put a heavy layer of newspaper down. This will plug small leaks if and when they happen.
Then I buy "Carlisle" roofing fabric, (rubber), and line the pond walls and floor with only 1 piece of fabric. You can splice, make sure you overlap at least 6".
I use only Osage pumps and heaters in the winter time to keep the water flowing from the water falls into the inlet to the pump.
The guy that showed me how to build a pond has ponds over 20 years old with no problems. I have used the roofing fabric on existing ponds with newspaper between the fabric layers, (old vs. new). Yes you have to spend some money in the beginning but when you divide the cost over 10 - 15 years, it's pretty cheap.