وَاضْرِبْ لَهُم مَثَلَ الْحَيَاةِ الدُّنْيَا كَمَاءٍ أَنْزَلْنَاهُ مِنَ السَّمَاءِ
Set forth for them the likeness of worldly life: it is like water that We send down from the sky.
In Qurʾān 18:45, the point is not exhausted by a literal equivalence of life and water. The broader image describes vegetation flourishing after rain and then withering and scattering. The condition of worldly life, its freshness and its transience, becomes intelligible through the whole sequence.
Thus a noun immediately after the kāf need not be the only component of the intended comparison image. The simile can concern an entire state and its transformation rather than an isolated substance.