Frosty,
Good advice. I actually have two separate articles on calf cramps at
Fitness and Exercise Articles Just for SCUBA Divers, one titled "Preventing Foot and Calf Cramps," and the other titled "Exercise Associated Muscle Cramping." Your experience matches that of the research, that stretching before an activity like finning can reduce cramping. This is thought to be in part due to the fact that stretching inhibits neuromuscular activity for a period, reducing the chance the calf, in this case, will overload and cramp.
As also pointed out, dehydration is an ancient theory from the turn of the last century that has been debunked by research done for the past 40 years, along with theories on electrolyte levels (e.g., "eat bananas for potassium"). Nobody is too bent out of shape over this, as increased attention to hydration and good nutrition won't hurt anyone. It just is not much of a solution to cramping.
Stretching cold muscles is just fine. While a warm-up is certainly the best thing to do when possible, you will not injure a muscle through gentle stretching. We have luckily been built a little tougher than that. <g> Dynamic stretching certainly does require a good warm-up to prevent injury, though this is due to its extreme loading of the muscle systems which is not done in static stretching.
Cameron