杯 is cup (or mug or glass) and is the measure word for water (and other drinks), and 一 is one, so to order a glass of cold water, ask for 一杯熱水 (yāt būi yiht séui in Cantonese, yī bēi dòng shuǐ in Mandarin), one cup of cold water.
About This Dish
Chinese restaurants often serve not just cold water but also hot water, basically hot tea without the tea. Drinking hot water is considered healthy for a combination of traditional medicine and water sanitation reasons.