Eight is a completely arbitrary number here. There is actually no evidence
to support having to drink this much water a day. You need to take in as much water as you lose in day and that can vary greatly for different people. Most experts come up with 4-6 glasses at the most. "83 ounces" seems completely insane. I couldn't drink 10 glasses of water a day if I tried.
What's even more interesting is this whole "Pop and coffee" don't count bit. The common claim is that these things contain caffeine which is a diuretic. The water lost because of this is very small though. If you're a regular coffee drinker it won't have any effect at all on you. On the other hand, you are getting extra sugar and other junk so plain water is better for you.
You actually get a great deal of water from the food you eat and don't really need to take in any extra. Drinking water does seem somewhat helpful when dieting though. In general, you should just drink something when you're thirsty and you'll be fine.
Thanks to the defenders of truth and justice, www.snopes.com.