If you have had a high blood sugar and have been drinking lots of water, you may not only have ended up being dehyrdated, but you may have also have caused an electrolyte imbalance. When you drink lots of water and then just have it come out the other end, it will leach out all kinds of things, sugar, potassium, sodium and magnesium. Even though you may rehydrate, you may well end up severely deficient in things like potassium. In fact, when they warn you against drinking too much water water (hypernatraemia), it is really this electrolyte problem that hurts you.
Take home message. If you drink large amounts of water, whether it be because of high blood sugar or just exercise, make sure you restore electrolytes. The best way to do this is to drink sugar free sports drinks that have a balanced electrolyte distribution. You can also make your own:
2qt Water
1 pack Sugar Free Kool Aid
1 tsp baking powder
1/2 tsp salt substitute (potassium chloride)
1/2 tsp salt
1/4 tablet magnessium supplement crushed