sighs*
drink just enough so that urine is not yellow in colour.

it is extremely hard to overdose on water unless the person got some psychological problem or is a aquaholic. overdosage of water may cause disruption to salt balance in blood, causing hypotonic of blood which lead to swelling of cells. swelling of the brain in extreme cases will lead to coma and even death.

i remember long time ago a marathon runner drinking water( that time no isotonic sport drink) at every station then end up fainting coma and death.

normal colour of urine is pale yellow.
basically, urine is excess water, salts and minerals, as well as urea from protein digestion, uric acid, creatinine from muscle breakdown, hormone waste and toxins. Also eliminated is urochrome, which is a yellow pigment that comes from the processing of dead blood cells in the liver.
the colour of urine indicates the concentration of waste product in urine. the darker the colour the higher the concentration of waste product in it.

for further understanding you can read how kidneys work:
http://health.howstuffworks.com/kidney.htm