Shower thoughts hit hard because nobody can reach you in there. Find a few more rooms like that.

Ideas turn up in the shower because it is one of the few stretches where nobody can reach you and your hands are busy with something automatic. A mind needs roughly that combination to wander usefully, and most of the day is built to prevent it. The water is not the active ingredient. Being unreachable is, and you can arrange more of that on purpose.

Look for other rooms like it: a walk with no headphones, the bus with your phone in your bag, washing dishes, the ten minutes before anyone else is awake. Pick one and use it a few times this week without filling it.