There isn't a lot of scientific evidence that supports 64oz of water per day. Some people are fine with less, and some need more. It can be very helpful in regulating hunger, and it aids in digestion, and of course hydration. It is important to get enough water, which is usually somewhere between 1 - 2 liters per day for most people. As long as your urine is clear or very light colored (or perhaps bright yellow from your multi-vitamin) you are probably okay. Obviously, if your urine is dark and/or you are having symptoms of dehydration, you would want to seek some medical attention and possibly get an IV.
I actually drink closer to 100oz per day, but I don't always get there. I am sipping constantly except for 30 min before and after meals. On days I don't get that much I tend to feel more hunger, but that's just me.
No need to stress about it too much unless you are doing exercise, sweating or have a health issue that passes water quickly through your system and increases your need for fluids.
As far as what counts as fluid. Of course, water is best, but you can reasonably count anything, although I generally wouldn't count drinks with a lot of caffeine. I think everything else is fair game, decaf coffee, tea, fruit essence water, natural sweetened, low or no sugar water, zero calorie sports drink, broth, etc. During the liquid and puree diet phase, you can also count you protein drinks and other liquids, like soup.
It gets easier over time for most people. It's a lot harder earlier in the process until your stomach/pouch relaxes a bit after healing.