But the quote is not talking about showers.

That would be like taking this quote: "We aim above the mark to hit the mark - Ralph Waldo Emerson", and say it talks about how to better your aim with the crossbow.

What's important in both quotes is the subtext, the hidden meaning. Paul is likely not talking about the power of showers, but about holding in your mind something you deeply care about—so much that you don't stop thinking about it even when you're taking a shower. Emerson is not talking about crossbows but about dreaming big.

You say you think about writing a lot while you shower, and that's because you love it. Try pondering something you don't care about and you'll end up hating shower time.

This piece is interesting, but relax; quotes aren't meant to be taken literally. This one is peculiar because showers do enhance creativity—but also taking walks, washing the dishes, and any physical, automatic activity that allows the brain to enter the daydream flow state you talk about.

Showers do not hold the power of anything. It's just the object Paul chose to focus the quote and make it beautiful. Maybe a little of both.