Overhydration!
Yes, you can have too much to drink! I have become increasingly aware of the problem of hyponatremia, which is occurring with greater frequency these days in marathon runners. Hyponatremia is the term for low blood sodium levels, clinically defined as 135 mmol/L or less. Hyponatremia is correlated to excessive fluid intake. Weight gain over the course of the marathon (or other endurance exercise event) is a predictable sign of hyponatremia.
During the past few years at various race expos we have attended, I have been surprised by the number of runners and walkers who are concerned about getting enough water to drink during their marathon. People ask me, "Can I carry my water bottle in the RaceReady LD (patented pocketed design) shorts?" Or, I get asked, "Will the pockets on the LD shorts interfere with my fanny pack?" I am confused with these concerns when they occur at the big, mass market running marathons, since with very few exceptions, water and electrolyte drinks are available every mile or two!
Of course I am not advocating dehydration! Dehydration is actually more prevalent among marathon runners than overhydration and is very dangerous, too. My point is that we need to be aware of how much water we are in fact consuming. Consuming no more than 7-10 fluid ounces every 15-20 minutes is sufficient for most runners under normal circumstances.
It comes down to knowing your body and it's individual requirements. Be aware of how much you drink during your training runs, and drink a similar amount during your race! It's not necessay to panic and gulp down large amounts of water every mile during the race.