The need for water during exercise is very much dependent on the circumstances and the environment.
It's worth reading www.sparkpeople.com/resource/fitness_artic
But to paraphrase, for most workouts under 60 minutes, you probably don't need to drink anything. For a short intense workout where you really are 'balls to the wall', you shouldn't need to drink. If you are working out for long enough that you need to drink, then you have time to drink.
Also, be guided by the conditions. In cool conditions you may not need to drink anything, but you may need to drink quite a bit if the weather is hot and/or humid.
Drinking 8 oz every 20 minutes may be necessary in some very warm conditions, but is NOT necessary in most other conditions. Incidentally, I read recently (can't remember where) that there is no recorded instance of anyone dying of dehydration in a race, but there are several deaths from Exercise-associated Hyponatremia (basically where essential body salts get diluted too much from excessive water intake).