@Perdido - Bananas are a good source of potassium, and I’m sure that eating them would do some good. I would think that you should also keep some sports drinks in your system, though, since keeping your body hydrated is super important if you want to prevent heat cramps.
My dad works in construction, and he downs sports drinks with electrolytes in them regularly, as well as water. We live in an area with really hot summers, and he knows that he has to protect his body from the effects of extreme heat.
One of his workers refused to drink much because it made him urinate so frequently, and he suffered some major heat cramps. They lasted over thirty minutes, and he even stopped sweating, so my dad rushed him to the hospital. He lived, but he had suffered a heat stroke.