Charlie, call them at least to ask? To be on the safe side?
I was NOT supposed to drink anything at all during my 1 hour glucose challenge. Yes prior to it (I was allowed to eat breakfast, too), but not during the wait.
When I had OGTT, the three hour thing, with fasting, many years ago, I was allowed to drink water prior to the test (no food, of course), but nothing at all during the three hour wait.