水 pictures water itself — a central flowing stroke flanked by splashing droplets. Where 川 is the riverbed, 水 is the substance. Identical across 繁體 / 新字体 / 简体. The most important radical-shape transformation in the entire script lives here: when 水 appears on the left of a compound character, it becomes 氵 (the "three-dot water" radical). Once you see 氵 you can predict the meaning involves water — 江 (river), 海 (sea), 湖 (lake), 泳 (swim), 汁 (juice), 浴 (bathe), 汚 (dirty/polluted). It is the single highest-yield radical to memorize early.

Mandarin: shuǐ, dipping 3rd tone — the pitch contour itself sounds like a small splash. 水果 (shuǐguǒ, fruit — literally "water-fruit"), 喝水 (hē shuǐ, to drink water), 风水 (fēngshuǐ, "wind-water" — the geomancy system Western readers know as feng shui), 矿泉水 (kuàngquánshuǐ, mineral water).

Japanese: on-reading スイ (sui) in Sino-Japanese compounds — 水曜日 (suiyōbi, Wednesday), 水泳 (suiei, swimming), 水準 (suijun, standard/level). The kun-reading みず (mizu) is the everyday word — 水 (mizu, water), 飲み水 (nomimizu, drinking water), 水着 (mizugi, swimsuit). Note that Wednesday is "water day" in the East Asian planetary week.

Memory aid: a vertical center stroke with two pairs of droplets falling outward — the picture is the meaning.