申し訳ありませんが、お水だけいただけますか？ — Excuse me, but may I just have water? (Izakaya · Restaurant, Formal — to staff, JLPT N3)

Gesture & etiquette

Politely raise a hand to signal a staff member, then point to an imaginary glass.

Some establishments (especially izakayas) have a 'one drink per person' policy. Be prepared to order a paid drink if water is not sufficient.