I quit having so much trouble with night cramps when I added a magnesium supplement.

A list of things that can help.
Magnesuim, potassium, water, massage, epsom salt bath, and dill pickle juice if you can't stomach the vinegar. There is an acid in the vinegar that your muscles run out of after intense physical activity, the vinegar puts it back.

If you don't have a foam roller for self massage, get one! They are about $15 at Wal Mart.

Yea if you're drinking A LOT of water you could be depleting electrolytes such as sodium, esp, etc. Also one banana may not be enough potassium, there are foods higher in potassium than a banana so you might try a potassium supplement but be sure to read the label. Magnesium may help you and it's infamous for it's ability to relax muscles but it does nothing for my deep muscle cramps in my calves, what has helped me is to be sure to stretch after exercise while the muscle is warm and don't miss a session.

Hydration, stretching, and enough minerals and electrolytes.