Several times now I have weighed myself before and after a shower, and noticed that my scale thinks I weigh less after taking a shower -- something like a pound less. Could that be right? Why would this happen? Does a shower cause dehydration, maybe? Which weight should I believe?
You will weigh less after a hot shower, and, you hit the nail on the head; sweating and dehydration. You're losing water weight in a hot shower and depending on how long and how hot your shower is, this number can differ.
It's similar to how MMA fighters sit in a sauna before weigh-in in order to make their weight class.
The important part here is this weight loss is incredibly temporary. You'll regain this weight after drinking water or even just getting your body back to room temperature.
You should aim to weigh yourself at a consistent time and in a consistent circumstance, maybe weekly. Your body will fluctuate weight throughout the day for many reasons.