When you lose large amounts of water through diarrhoea, vomiting or drinking excessive amounts of alcohol (as it has a diuretic effect), along with the water you are losing electrolytes which are needed by the cells to actually retain and use the water.
Without the electrolytes you could drink as much water as you want but it wouldn't help. To get electrolytes you could eat an apple or other fruit/vegetable, but I buy a pre mix drink of electrolytes from the pharmacy because I tend to drink a lot and it's probably the most effective way to replace the lost electrolytes.
