Sip of water is fine but not too much as advised from gynae and nurses, you can drink in between while taking the test as well. Just make sure you don't vomit after taking the sugar drink because you will need to redo it.
The nurses told me no plain water also.. cos will effect my results then i will have to come back again for a retest. So i really just controlled tho i was really giddy by half the day.
For OGTT, you can only have plain water. Only after your last test then you can have meal and other preferred drinks. This is to avoid interruption of reading of your OGTT result.
Sip of water is fine as advice from gynae n nurses. Not too much of water as will affect the test.