there is a reason but your doctor has apparently not done a thorough diagnostic work-up to find the cause. while drinking water is always advisable but if this has been your only intervention and it is not working it is time for you to return to see your doctor again. there are sooooo many reasons to account for your blacking out and the lack of personal medical information that it would be impossible to make that patient-specific determination over the internet.
i black out for an unknown reason. my doctor told me to drink water so i have been. (2 months) nothing is working. . when blacked out can't hear that well lose my balance last any where from 30 seconds to 5 minutes.