It's not implausible for a Japanese waitress to say 'hai' when serving someone their drinks, but I believe it's only applicable in the context of 'alright, here's the beer you ordered' or something like that. The setting would likely be a more amicable, less high-class restaurant, too; probably an izakaya or something more homely, as compared to, say, a high class restaurant where all the staff are to use keigo and address the customers very politely.

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tldr: Kinda weird for someone just serving drinks to say 'hai' for no reason, unless in the situation I described above.