General:
* Water streamed down like silk.
* Water streamed down like tears.
* Water streamed down like molten lava.
* Water streamed down like a waterfall.
* Water streamed down like a river.
More specific:
* Water streamed down the windowpane like a thousand tiny fingers.
* Water streamed down the roof like a silver curtain.
* Water streamed down the hillside like a cascade of diamonds.
Figurative:
* Water streamed down the mountain like a serpent writhing its way to the valley.
* Water streamed down the canyon walls like a painter's brushstrokes across a canvas.
The best simile will depend on the specific context and the effect you are trying to create.