すみません、お冷はいただけますか？ — Excuse me, may I have some tap water (cold water)? (Izakaya · Restaurant, Formal — to staff, JLPT N5)

Gesture & etiquette

Make a subtle drinking motion, or just politely wait for the response.

In some izakayas, especially cheaper ones, ordering a paid drink is implicitly expected. While it's acceptable to ask for o-hiya, try to also order at least one paid drink per person if you are staying for a long time.