すみません、お水、もう少しいただけますか？ — Excuse me, could I get a little more water, please? (Restaurant · Izakaya, Formal — to staff, JLPT N5)

Gesture & etiquette

Gently raise hand to call staff, then use a small hand gesture towards your glass.

Do not make loud requests. Be polite and wait for a good timing to call a server. Some places might charge for refills, though less common for water.