1. Divots like that under a liner could be caused by pinholes.

2. Odds are, if the pinholes ARE the installer's fault, he'll lie about it.

3. BUT that doesn't mean that they ARE his fault. It just means that the installer's denial doesn't tell you anything, one way or the other.

4. If the pinholes are caused by abrasion UNDERNEATH the liner, then the installer is probably at fault. If the pinhole is caused by something ABOVE the liner, then you are at fault, or at least, the fault is your responsibility. (You haven't had any bored ill-disciplined 12 year old boys in your pool, have you? I've seen an awful lot of sneaky vandalism at commercial pools!) If the pinhole seems to be IN the material of the liner . . . then the problem is probably LINER MAKER'S fault.

5. Figuring out which is which will require looking closely at a pinhole. Dye-testing will help you pinpoint a pinhole: http://pool9.net/dye-test