I wouldn't let them off the hook. You paid for the installation to be done correctly.

When I installed my first liner years back, we did it ourself. We got the pool up and the liner in and then threw the hose in to start filling it up. It was going fine. We had duct-taped the hose to the top rail so the hose would stay in place. Throughout the night, we got up every couple of hours to check the hose. One time when we got up, we found that the duct tape had popped loose and the hose had fallen to the ground and was aimed at the base of the pool. It had washed out a footprint sized area of sand about 1-1/2 inches deep that was about 2 inches away from the pool wall on the bottom of the pool. I was so disappointed. My new pool wasn't perfect. But, we told the kids to leave the footprint alone and it never got any bigger. It wasn't an issue with the integrity of my pool, but was a cosmetic issue. However, WE were at fault, not an installer.

Your case is different. You hired someone to do the job correctly.