Martin, as ummmm says if your urine is light in colour, you would not be dehydrated, I have learned over past year to drink at least 2 ltr water - works out 1 pint and bit more around meal times so it easy remember, I am a divil for coffee so it has been suggested I add more water to my recommended daily dose because coffee to the extent I was drinking it, dehydrates. I fill one of those big hydrate bottles in morning with fresh water and by bed time, it has to be empty. After that, I can't say why your dizzy, sounds like BP - if your near a pharmacy, get it checked out if you not really fancying go to gp. Keep strong xx