When I say rocks, I mean boulders. Small rocks have their place, and they should he used, but bear in mind that they can easily he overgrown and would become lost. Except with respect to streams, where placement of rocks is governed by the effects you want to create, rocks should look as natural as possible. One rock goes where each potato has landed. Water, or the lack of it, is an essential part of any landscape.
Water delights the eye whether still or moving.
Moving water provides a number of interesting sounds for the ear. Three of these are basic. Shallow water moving at a moderate rate over small stones creates a sibilant whisper. Deeper water moving rapidly over larger stones roars and pulses. Heard at a slight distance, the effect is of someone speaking quietly, but not understood.