"He has already showered today."
Translation:Он сегодня уже мылся.
"Он сегодня уже принял душ"... The "correct" answer translates to "he already cleaned himself today".
Мыться is commonly used with this meaning "to shower" (you do clean yourself when you shower, don't you?). "Принял душ" maybe should be accepted but the other I think is the more common way to say it.