On the desktop version of the website, if I click on the listening icon at the top of this discussion, it says すいようび (suiyoubi). Are you talking about how if you choose the individual characters to input your answer that 水 is read as "mizu"?
The whole system of splitting words/characters apart and giving them only one pronunciation doesn't really work here...
Yeah, Wednesday is literally water day. All of the days of the week contain a particular kanji like that.
Sunday: 日曜日 (literally "sun day"- the only one that works as a literal translation from English)
Monday: 月曜日 (literally "moon day")
Tuesday: 火曜日 (fire day)
Thursday: 木曜日 (wood day)
Friday: 金曜日 (gold day)
Saturday: 土曜日 (earth day)
Blame the Romans. The Chinese got this from them I think (and the Japanese got it from the Chinese).
The modern Latin languages still use this for all the days. e.g. in French:
Mardi - Tuesday (Mars day - element of fire)
Mercredi - Wednesday (Mercury day - element of water)
Jeudi - Thursday (Jupiter day - element of wood)
Vendredi - Friday (Venus day - element of gold)
Samedi - Saturday (Saturn day - element of earth)
We changed Tuesday to Friday in English and named them after Scandinavian gods instead.
Like Monday the day of the moon or Sunday the day of the sun, every day is relative to a planet in every Latin languages. In japanese days are relatives to elements
Sunday、日曜日、day of the sun
Monday、月曜日、day of the moon
Tuesday（Mardi）、火曜日、day of the Mars
Wednesday（Mercredi）、水曜日、day of the Mercury
Thursday（Jeudi）、木曜日、day of the Jupiter
Friday（Vendredi）、金曜日、day of the Venus
Saturday（Samedi）、土曜日、day of the Saturn
Kanji have what's called an on reading (the Chinese sound) and a kun reading (the Japanese sound). The kun reading of 水 is "mizu". Water by itself is 水 (mizu). Drinking water is 飲み水 (nomimizu). The on reading is "sui". Wednesday is 水曜日 (suiyoubi). Swimming is 水泳 (suiei).