Add some electrolytes to that water. A bit of salt and potasium should do if you eat regular food for your meals(some people do the liquid diet thing).
Sweat contains a lot of salt and potassium (it's slightly less salty if one is accustomed to hot weather riding).

When the blood electolyte drops too low, the cells and he brain still contain electrolyte and start aborbing water from the blood in order to balance the electrolyte levels, this is due to something called osmosis. Because it is absorbing water the brain physicaly swells this is not a good thing, severe cases have killed athletes who drank plain water for a competition then chugged a bunch more plain water right after. Less severe cases have symptoms similar to dehydration.

Under less than racing conditions your body will continue to produce urine and balance the water to salt, unless one is dehydrated(the body considers dehydration worse than salt imbalance) in which case you get both problems. However, neither water or salts can be produced by the body and there is a limit to urine salt consentration (high and low) though it it is a fairly wide range, so both elements, water and electrolytes, must be consumed in a reasonable ratio over the coarse of the day.

I used salt and electrolyte interchangably here but electrolyte generally covers sodium, potasium, magnesium, calcium, and some very minor elements, chloride could be thrown in for this sweat dicussion to.
Table salt, rock salt, sea salt, etc. are all 99% sodium chloride. Sweat mainly contains sodium and chloride, but also has a good amount of potassium (can be met by high K food like bananas and oranges), and a small amount of the others (which all store in the body much better than salt, so standard good food is generally adaquate).

So, unless your racing or close to it, where you can't take in much real balanced food,(in which case I would recomend a fancy blend like "Endurolytes") I would recomend simply adding a teaspoon[5cc] of iodised salt per quart/liter of water and eating right.

Those water IV drips at the hospital always contain salt, hense the fancy term "saline solution". Often a touch of glucose as well, though very small.