**”A shower water valve with a pushbutton on/off switch. I have no problem with shutting the water off whilst soaping up and then turning it on again to rinse (this was required for showers when I was on a submarine)...except that it sucks having to readjust the cold and hot water to get the temp right when turning the water back on—to the point where I don’t turn the water off.”**
That is a common $5.00 and $20.00 item, and is often free.
https://www.google.com/search?newwindow=1&es_sm=93&biw=1440&bih=813&tbm=isch&sa=1&q=shower+head+with+flow+button+button&oq=shower+head+with+flow+button+button&gs_l=img.3...31847.38541.0.38829.24.23.0.0.0.5.551.3402.2-3j3j1j2.9.0.msedr...0...1c.1.64.img..23.1.550.edfTO8AE-Qg
Thanks....hadn’t seen those before. Will have to go get one...
I still like my thermostat idea though...then you don’t have to play with the valves yourself, wasting water in the process.