just drink the shower water. that way you can save time by not having to pour a glass of water when you are done.
The soaps, hot water, and other mineral could possibly evaporate a lot of water from your skin, semi-dehydrating you.... That's an absolute guess though.
I say it's purely in the mind.. you're probably thirsty even before you step into the shower, but it doesn't occur to you until you're reminded by the sight and sound of all that water. Just like peeing and waterfalls