Potassium seems to be it for me too. V8 juice, especially the Low Sodium has a bunch and has cured my cramps.As a former "way back when" Iron Man, POTASSIUM is the key.
Currently do minimal diving here in Ontario, when I do duration is relatively short so no prob.
My passion right now is MX caves, spend 3/4 weeks down there doing 3.5/4 hour dives and it is rare for me to not experience calf/leg cramps in the first few days.
Stretch them out and dive on.