this ‘danger’ is extremely rare and will only occur if you combine very long instances of exercise with drinking a lot of water without replenishing electrolytes at the same time. The term is ‘hyponatremia’.
If you drink, let’s say, 4 cups of water while you exercise you are not in any danger. But if you decide to drink 4 gallons over a short period of time, you could run the risk of ‘water intoxication’.
Stay well and drink responsibly (even water).