I almost never drink straight water for a ride. I've always got a mix in my bottles that includes a little bit of electrolytes. However for those that do drink straight water, they probably should start thinking about a source of electrolytes when going from their third bottle to forth bottle of straight water. Because after the fifth bottle of straight water they can be in dangerous territory if those five bottles are consumed over a relatively short period of time.
The electrolytes don't have to be in your bottles. You can just eat something or take a tablet. Plain table salt is an electrolyte. Sodium, calcium, potassium, chlorides, phosphates and magnesium are all electrolytes. Some more than others. But even a salty snack along the way will help. I just prefer to put it in my bottles like I do my carbs. It's just too bothersome for me to pull out a gel or pill while riding. Not that I can't do it, just that I don't care to since bottles are more convenient.
However more is not better. So be careful not to get into the idea of just simply adding huge amounts of electrolytes into your hydration/feeding regimen while riding. I don't use Gatorade, PowerAid or other pre-made sport drinks because I feel they have too much electrolyte. At least for me they do.