It's almost certainly not a hydration issue. Sounds like you were drinking plenty, and besides, insufficient hydration/electrolytes isn't actually that well supported as a cause of cramps. See this Science of Sport series: http://www.sportsscientists.com/2007...ories-and.html.

I agree with the poster who said it sounds like your fit is off. I would add to that the question of overall fitness. I would take a look at your cleat position and/or saddle height. If you're putting a lot of strain on your calves because of your riding position, that would likely contribute to cramping. My first thought is that you should consider moving your cleats back so there's less of a lever force on your foot that your calf has to compensate for. Good luck.