I don’t think it is possible to race a marathon (I use the word ‘race’ as against ‘run’ or ‘complete’ because ‘race’ to me means pushing as close to your limits as you can) without getting dehydrated.
As with so many things relating to a marathon, the key here is making sure you get it right in the days leading up to your race, so drink plenty in the few days before the race – and if you have to go to a race expo and walk around for hours looking at the latest gadget or pair of shoes, take a BIG bottle of water with you: a few sample cups of energy drink is not sufficient. Then on the morning of the race sip something like water or a diluted energy drink and make sure that you empty your bladder as close to the start of the race as possible.
During the race
Quite simply the act of getting water in your mouth when you are running as fast as you can, is not easy and at best you are likely to only get a mouthful or so. And even if you can get water into your mouth, your stomach can only absorb a certain amount and you really don’t want water sloshing around inside you as you run. So ‘little and often’ is my best advice here.
It has been reported that the great Haile Gebrselassie was 9% dehydrated when he set the then world record of 2:03:59. And his last mile was astonishingly fast. So dehydration can be managed. Just take a mouthful as often as you think is necessary, don’t drink too much and make sure you are very well hydrated before you start. Now that’s not complicated, is it?