The movement of water.
We find the gentle flux and reflux of water as if it were lapping a rocky shore in the exquisite figure out of which Mendelssohn constructed his "Hebrides" overture:
and in fancy we ride on mighty surges when we listen to the principal subject of Rubinstein's "Ocean" symphony:
In none of these instances can the composer be said to be imitative. Music cannot copy water, but it can do what water does, and so suggest water.