I tried potassium, sodium, magnesium, etc., supplements--no improvement
Putting on my physiology hat here....electrolyte deficiencies, such as potassium, can contribute to muscle cramps. But, cardiac muscle contractions rely on similar electrolyte concentrations. Thus, electrolyte deficiencies severe enough to generate skeletal muscle cramps often generate cardiac arrhythmia as well. Regular muscle cramps are much more likely to occur because you run out of the "fuel," (eg. sugars and fatty acids) necessary to generate ATP. The energy molecule ATP, powers muscle contraction, but it is also required to "unlock" muscle fibers after the contraction cycle. Thus running low on ATP is a contributor to muscle cramps because a low concentration of the molecule in muscle fiber contributes to the inability of the muscles to relax. Folks often say that bananas reduce muscle cramps because they are rich in potassium. It's far more likely that just the additional fruit sugars help. So your best bet is to get a good fitting fin that is not overly stiff for you, making sure that your legs are well conditioned, staying hydrated and making sure that you got plenty of carbs the night before and morning of diving.