It's done differently in different places. Everyone should follow the instructions given on not fasting or fasting because it's likely that the lab values differ and will change with how the test is performed.
When i lived in central pennsylvania they had everyone get tested at 16 weeks and again at 28 weeks. (mostly bc there is a high rate of diabetes there). Anyways, they made me fast for 12 hours before hand. The drink was around 10oz. I only had to do the one hour since i passed. If i would have failed i would have had to do the 3 hour.
Then we relocated to coastal New Hampshire where i went to an OBGYN/CNM office for the rest of my pregnancy with DS1. I did the 1 hour test at 28 weeks. Same deal as the other place (12 hours of fasting, 10oz of drink, 1 hour test).
Then, for DS2 i went with HB MWs. I was offered the test but decided since i had a meter and knew how to check sugars, i just did that a few times. I felt comfortable with that. I had the option to do the GD test (the 1 hour, but declined).
With this baby I am going with those same HB MWs but this time i am going to test. It was offered. Same deal as last time and i decided to take it. I do not have to fast, at all. In fact the MWs said eat how you normally would and dont avoid certain foods due to sugar (that you would typically eat) bc that can give off a false positive. So eat just as normal. Dont fast, dont avoid foods you generally eat (but also dont binge on sweets). Drink the 10oz thing and do the one hour in their (out of hospital) birth center. So even though we still live in coastal New Hampshire, it's just different bc it's a different practice. As well im sure it has a lot to do with the numbers in the lab, when they figure the results which is why it's so very important to do exactly how you are told in reguards to fasting or eating normally etc.
RE: drinking water. It's actually encouraged. If you dont have enough water in your system, THAT is what can give a false positive. Drinking water is always encouraged bc that gives a more truthful result and it makes getting the blood draw easier (more water = better blood volume). Just remember your body is made up 75% of water, so drinking lots of water gives a better picture. Drink to thirst.