What does a water pill do and how do I know if I need one?

Diuretics. Divided into classes by part of the kidney they act on. They are used for fluid overload as in CHF or to lower BP. There are weak diuretics like hydrochlorthiazide or stronger ones like metolazone. Consult your doctor before taking these medications.
Diuretic. "Water pills" are lay terms for diuretics. These are several, but all block the kidney from re- absorbing sodium, which allows the body to eliminate excess salt and water. Diuretics should only be used when there is evidence of salt and water overload, as in heart failure, kidney failure, liver failure, and in certain patients with high blood pressure. They have numerous side effects. .