In creating water features of unlimited shape and size, a liner must be flexible enough to bend and turn to fill every void, course over every curve.  That liner must also be very durable, to support the naturalization of the feature by using literally tons of pebbles, stones and boulders.  It must also support thousands of gallons of water.  It must be made out of materials such that fish can safely swim in the water cupped by the liner.  That's why we use 45mil EPDM fish-safe liner.  It's thick, flexible yet durable, and perfectly suitable for our finned friends.

And under that liner we use a dimensional underlayment fabric, design to allow the escape of gases from the soils beneath the water feature, and to protect the liner above from sharp objects below.