In a good workout you lose 1-2 pounds of water. We had a scientist in our group who I haven't seen in a while, otherwise I would ask him, who explained it to me once but I forgot the explanation. It had something to do with osmosis (it is not perspiration in the usual sense), and fluid leaving your body in an attempt to achieve equilibrium with the water you are immersed in. It is the same process that makes fingertips prune up.
What I'm wondering is what this loss of fluid does to the sodium/potassium balance and is it related to getting cramps after an hour or so.