The glyph origins of 水 resemble a flowing stream. 氵 is the left radical form of this character, so if you see a character with that on the left, chances are it has something to do with water or liquid, e.g. 酒 liquor, 油 oil (the edible kind) or sauce, 海 ocean, and 清 soup.
Ordering
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 water, ask for 一杯水 (yāt būi séui in Cantonese, yī bēi shuǐ in Mandarin), one cup of water.