Chinese · Pronunciation Guide
shway, with a retroflex sh and a low dipping pitch. The tongue curls back for the hiss and then has to travel a long way forward for the vowel.
shuǐ
One syllable with a third tone, so the sandhi question is entirely about what follows. Shuǐ guǒ, shuǐ jiǎo and similar third-third compounds push it up to a rising tone; before anything else it keeps the low dip. Alone, as in a request for water, it gets the full dipping contour.
Listen: 水
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我想要一杯水。
Wǒ xiǎng yào yì bēi shuǐ.
I would like a glass of water.
Listen: 我想要一杯水。
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我想要一杯水。 finishes on shuǐ, which is where the third tone can be at its fullest — nothing follows, so the pitch dips and curls up. Earlier in the sentence, wǒ xiǎng puts two third tones together and wǒ rises accordingly. And yì bēi shows 一 already converted to a falling tone ahead of the first-tone bēi.
The standard error is a flat English "shway" with the tongue tip down and the whole syllable said at one pitch. Curl the tongue back until the sound feels slightly muffled, then let the pitch drop. Learners also frequently drop the middle vowel and produce "shwee", which loses the characteristic three-part glide and makes the syllable sound thin.