Supercooled water isn't quite the same thing you're asking for - I doubt it could be done with a shower without special equipment - but here's a video of spontaneous freezing in two ways: https://www.youtube.com/watch?v=Fot3m7kyLn4

The first, pounding it inside the sealed bottle, is creating some sort of disturbance to crystalize around (an "unsuspecting nucleus"), and the second is pouring it semi-shower-like.