Is drinking water too much is toxic/harmful?

Drinking too much water dillutes your system and hurts your body's ability to filter out toxins. The limit varies on the person and their own life. You have to know your own needs. If your pee is very dark, then you're not drinking enough. If it's pretty much colorless, you're drinking too much. Simple.

I don't know how it is reliable but I read an article of drinking too much water on a newspaper. It said that drinking much of water in a day is a good habit. It will provide your body sufficient amount of water needed and refresh your blood. But if you drink too much water, there will a disadvantage that your kidney will have to work harder to filter with large amount of water and later on its functions will be affected. The suggestion is drinking right amount of water that your body needs 2.4L/each day, perhaps. To me, one more problem is when I drink too much water than I get to visit toilets more frequently, haha.

Yes, drinking a lot of water very quickly is deadly. It causes an electrolyte imbalance and symptoms similar to drunkenness. It is very difficult to drink so much, though.