How do you simulate the warm water?
Doing any relatively rote act like washing dishes, walking places, etc can also give rise to them. Not having a device in your hand to constantly steal your attention really helps though.
Couldn’t the relaxation be a factor in generating shower thoughts?
I suspect that essentially none of our non-ancestors were predated in a hot spring, unlike walking etc, so there may be an environmental cue driven induced relaxation that doesn’t exist for many other activities.
I suspect it's just about getting the space to relax, which is why I frequently have thoughts when staring at the wall, or taking a walk, or washing dishes, or doing any other myriad activities which are relatively easy on brain processing.