Waterline (or perimeter) tile on a fiberglass pool is actually the same as in a concrete pool, ceramic or glass tiles are adhered to the surface of the pool with a specially formulated silicone product. These tiles are grouted with a silicone product as well, allowing the tile to flex and move with the pool shell.
However, the number of tiled fiberglass pools has decreased with the advent of colored pool finishes. The main benefit of tile continues to be scum-line control in addition to its aesthetic appearance. However, the scum line control is not a necessity in a fiberglass pool as in concrete pools. It can still be just to accent the waterline very nicely and make the perfect transition between pool and deck. Unlike a concrete pool, a fiberglass pool doesn’t need waterline tile to keep the waterline free of staining. It’s merely for aesthetics.