I felt so nauseous in the mornings that I couldn't even look at my toothbrush without gagging. Drinking water first thing was the only way I could calm my stomach, so go ahead.
Of course you can, actually it's very normal. If brushing first thing triggers your vomiting, just drink a glass of water first and brush later when you feel settled.
Yes dear, it is completely fine to have water before brushing, especially if you feel dehydrated. Please don't worry about these small things, you are doing great.