Stew Smith claims you're suppose to drink the typical 8 glasses of 8oz... plus one more glass per 10lbs of weight you want to lose if you are actually working out. He's a former navy seal turned PT. Has serious ideas about weightloss LOL But that was the only example of a PT that says drink tons of water.
If you are working out heavy- and seriously sweating, grab a propel fitness water or gatorade. The electrolytes JayEll mentioned- if you are eating right, working out a moderate amount *not four hour binges of working out lol* then water is perfectly fine. The issue is that if you sweat all day, either working outside soaked in sweat and then go to working out, (having an outside physical job)- water is NOT going to be enough for you.
Also, make sure that you are actually expelling close to what you're putting in- water intoxication is rare but serious- if you're spreading your water out over the 18 hour period you're awake and peeing almost as often- you should be alright
Hope that helps