Interesting question
This is an interesting thing to ponder.
Somewhere, maybe on this forum, I read that tile is like paint--it will look only as good as the surface onto which it is applied. I read this too late to save my own water line.
We just went through a pool renovation. Professionals installed our tile, which is small 1-inch by 1-inch tile. They did not take care to have a perfectly even surface beneath the tile. Most of the tile area is almost flat, but there are a few linear feet that are just embarassingly bad. We were too worn down to have it redone. Many things had to be redone, so the tile was not one of the battles chosen.
I am saying all this here because it is possible that a DIY pool owner may actually take more care to do the job well than a pool builder's crew might do.
Just because someone is a professional doesn't guarantee that they will always do a stunning job. It doesn't matter how many gorgeous pools I saw before going with my builder.
Lana