I'm not sure how solid any of the science behind any of this is but the following listed remedies may help: (Gleaned from my perusal of medical webistes that don't suck)

Water: Dehydration

Electrolytes:
Magnesium
Potassium
Calcium

Potential remedy: Bottle of Poweraid and then the power aid bottle filled with water (I understand that triathletes use this one?)

There's also a listing for both hyperventilation and lack of blood supply to the perhiphery which !might! be factors in UWH. The Mayo clinic also lists some medical disorders (thyroid or hormone disorders, diabetes, hypoglycemia and anemia.)

My own experience is that being fitter helps keep cramp away. I used to get it when I was a spag and I get it again now I'm a fat desk surfer. In the middle when I was fit it wasn't really ever a problem for me.