I think the advice not to bathe or shower after toking is a distortion of another type of don't-do-this advice having to do with another substance, guys. That's combining drinking (we can't go into detail on this subject here since it's not cannabis, but this is fairly standard medical advice) and sitting for any significant length of time in a hot tub or hot bath. Google "alcohol in hot tubs" and read what the many bits of information say there. It's a fast way to combine two ways of dilating your blood vessels with dehydration, and it can result in harm, particularly to people with pre-existing cardiac, circulatory, and/or diabetic conditions.

I suspect there may be some element of similar risk when combining over-vasodilation and dehydration resulting from a heavy stone with hot-tub sitting, but I'm sure there've been relatively few official reports of problems in this area simply because of the mostly illicit nature of cannabis. Anyone who's ever worked in any branch of emergency-related medicine/paramedicine sees folks regularly who've had too much to drink and then over-done it in a hot tub and either slipped under the surface and drowned or lost consciousness or had a heart attack.