But there is nothing wrong with Hook grabbing a drink. And it’s not like he was drunk. He could grab a drink regardless of the situation and it would be fine.
I wonder who the other drink was for, before Emma showed up. Hook might’ve wanted two drinks for himself, but there was room in each glass for about five times what they held. I’m guessing he was asking Charming for advice; Charming would have quietly left if he’d returned from the bathroom to find Emma and Hook patching things up.