cracked head is usual cause
usual cause is down to someone over tightening the spark plugs
put some of the water on your finger and rub it between your fingers, it feels like it has an oily feel to it and when it drys up it is sticky then it is coolant
ask yourself where will the coolant come from
heard of cam cover gaskets leaking allowing moisture buildup and eventual water but for it to fill up then what else could it be
u need a new head if it is cracked and i wouldnt drive it around much with a cracked head otherwise it will start to seize up the bottom end if leaking through