Ask the tile guy how he intends to prep the surface. You MUST get all the loose (de-lamed) plaster off. It may mean chipping the entire pool. Tiling is expensive because its so labor intensive, so this is going to be a major investment. Unless you have some experience in tiling POOLS, not just your kitchen back splash, have it done by a tile guy with references, or better yet, some pools that he has done, and not necessarily recently. You are looking to see how well his work is holding up.