Going without tile is not all that common, but it can work fine in all but the hottest climates. It is important to keep the plaster damp while it is curing, but after that plaster can be fine above water. The only significant exception is in very hot sunny climates (Nevada for example) thermal stress from day to night can be too much for plaster. Tile has a few minor advantages. It is easier to clean waterline scum off of tiles. Most people like the look, for example liners often include a "tile border" in the pattern.