Water in the body is located within the cells, in the space surrounding the cells and in the fluid component of the blood, called the serum. In answer to your question about passing out, fainting is a symptom of an inadequate supply of oxygen and nutrients to the brain, caused by a temporary decrease in blood flow. One cause of decreased blood flow is low blood pressure, and one cause of low blood pressure is low blood volume due to dehydration. So, yes, dehydration can cause fainting. Diuretics are drugs that cause the kidneys to excrete excess amounts of water in the form of urine.
Diuretic use is an important cause of dehydration.
Electrolytes are also often lost in addition to the water in dehydration. Electrolytes, or salts as they are sometimes called, are necessary components of an overall nutritional package required to sustain life. They are charged particles of simple chemical compounds, dissolved in the fluids. The most common electrolytes in the body are sodium, potassium, calcium, magnesium, chloride, phosphates and bicarbonate. Each plays an important but different role in body function.