French · Pronunciation Guide
loh — just two sounds. Those four letters "l'eau" produce one short syllable, nothing more.
A single syllable, so the word carries whatever stress its phrase gives it. Since the article is fused on, there is no separate beat for "l'" — do not let it become "luh-OH".
Listen: l'eau (f.)
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Je voudrais un verre d'eau.
I'd like a glass of water.
Listen: Je voudrais un verre d'eau.
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"Je voudrais un verre d'eau" repeats the same elision trick with a different word: "de eau" becomes "d'eau", because French will not tolerate two vowels meeting across that boundary. So the sentence has both "l'eau" as the headword and "d'eau" in use — the same rule, two different little words. Note also "voudrais", the conditional, which is what makes the request polite rather than blunt.
The spelling "eau" throws people badly, because English has nothing like it. Learners guess "ee-ow", "ay-oh", or a two-syllable "eh-OH". It is one sound: "oh". The second problem is the apostrophe — English speakers give it a beat, saying "luh-OH", when the article has been deliberately compressed to avoid exactly that. It is one syllable: loh, rhyming with "low".