As for the hydration... be sure to start it the day before and not wait until the day you are diving to start drinking water. If you wait until the last minute, most of that will pass right through you.
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In general, a healthy person who is not on any medications and who is eating a varied diet is not going to be deficient in potassium. Potassium is ubiquitous in food, and the kidneys mostly have to get rid of excess, rather than trying to retain it.