A quick idea.
The reason for a dedicated shower clothes is to preserve water. I believe its the heat from the water that prolongs its use.
So the idea is to have a special bading suit. To preserve heat and then also water.
So here is a product opportunity. A shower suits for men and women that preserves heat, dries up quickly and easy to wash in.
Or come think of it. Looking like a Borat swim suit might not be so popular. So maybe a ?larger shower towel or some like that works.