すみません、お水（お冷や）をいただけますか？ — Excuse me, could I get some tap water (cold water), please? (Izakaya · Dining, Formal — to staff, JLPT N4)

Gesture & etiquette

Hand gesture to ask for a drink.

In some restaurants or izakayas, tap water may not be automatically provided or might come with a small charge for service (rare for plain water, more common for o-toshi). Do not assume it's always free.