The Shit Creek Paddle Store

I've always wondered about that saying "up shit creek without a paddle", why would you even be on a creek of shit? Where would you find a creek of shit? Wouldn't a shit creek also be known as a sewer? If you lost your paddle, there would be other things in the boat that you could use, say the lid to the beer cooler (there has to be alcohol involved, why else would you be paddling around in shit?) If you're on shit creek, is it considered bad etiquette to piss over the side of the boat? I'm sorry, but if you're stupid enough to go boating in a sewer, you deserve whatever happens.