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.
And in English, the days are based on the Norse pantheon to a large extent.
Tuesday - Tyr's day
Wednesday - Odin's day
Thursday - Thor's day
Friday - Freya's day
And Sat, Sun, and Mon are Saturn, Sun, and Moon, as with Romance languages.
Interesting that the Chinese may have adopted this from the Romans. Contact was pretty limited, with Persians being incentivized to play middle-man and impede direct contact. I suspect general associations (not to specific pantheons... elements, more likely) may go back further than that, however.
It's common in writing to shorten the days like that, and it's also pretty common in casual conversation. For example, dates are usually written as 3月4日（水）. In speech, someone might be talking about what days they have off from work and say something like 水・木・金は休みです (sui, moku, kin wa yasumi desu, I have Wednesday, Thursday, and Friday off).