Ok, given that one cubic cm (=1ml) of water has a mass of 1 gram (I don't have my Merck to check, but that's what they originally based the gram on), and that there are 236.6 ml in 8 ounces, we would put 8 ounces of pure light water at 236.6 grams. With a molecular weight of 18 (O=16 + H=1 x 2 atoms), there are 13.14 moles of water. Multiply by Avogadro's for 7.913 x 10^24 molecules of H2O. With ten protons per molecule, we get 7.913x10^25 protons in an eight ounce glass of water.

I don't know to what extent isotopes of oxygen and hydrogen in the water would effect the molarity calculations, or if they even do. At any rate, given that the eight ounce glass is theoretical (ie, no measuring or contamination concerns), but even with isotopes effecting the calculations, I would still be confident in two significant digits, ie 7.9x10^25 protons.