if you want a quick fix and will do an excelent job use polyurethan caulk behind the new tile you are relacing and stick it on the gunite. It will grab untill dry and you wont need to worry about thinset falling in the pool. then just grout it with a small batch of grout. I did the tile this way when i was doing the tile on my pool to skimmer roof, it's not easy to keep the tile from wanting to fall down with thinset, even if you were to make a brace for it. I use Polyurethane everyday for work doing windows and doors, stuff is awesome! you can clean up excess with a lacquer thinner