Do I need to use underlay?

If you want to keep your liner protected and avoid unnecessary leakage, a durable underlay is key. Some people have used newspapers and old carpet as underlay in the past, but this option isn’t 100% cost-effective or secure. I totally do not recommend using anything except a quality underlay material. 

A proper underlay material adds protective barrier between soil in the ground and the pond liner. This extra layer will reduce friction wear on the underside of the pond liner.

For quality results, it’s in your favor to choose a geo-textile fabric. This will keep roots and sharp stones from breaking through the liner, and can save you money in the long run. After all, the last thing you want is have a leak in your liner after the pond is complete. 

Like I’ve said many times before, an ounce of prevention is worth a pound of cure.

Garth Epp Publisher, BuildingMyPond.com

In a word yes.  You must protect the liner from sharp rocks ect. that do move in the soil with the frost ect. Now that being said you do not need to buy it from a pond supplier , it is just easier to do one stop shopping . What can be used is old carpet, hard to work with, or heavy wait landscapers cloth or anything else that will give that protective barrier.  You still must use clean sand a few inches thick to line the hole. Also don't forget to protect the liner on the top side where you place rock ect. A little extra work building will save a lot of greaf later and be more enjoyable .