すみません、お水いただけますか？普通の水道水で大丈夫です。 — Excuse me, may I have some water? Tap water is fine. (Izakaya · Dining, Formal — to staff, JLPT N4)

Gesture & etiquette

Mime drinking from a glass, or simply make eye contact and a slight bow.

In some establishments, tap water might not be offered by default, or they may offer bottled water for a fee. Be polite and understand that it's not always free or standard. Avoid demanding.