I agree with you, how's the river and what's the river like are pretty much the same.
"Milyen" is a question word that means something like "of what sort?" The duolingo translation seems to be "What is ... like?", which is pretty close.
Come to Kisoroszi and have a good swim and I will ask you "How is the river" not "what is the river like"... (Kisoroszi is a small village upstream Budapest, and has nice places where you can swim in the Danube quite safely.)
If you were swimming and asked "Milyen a viz?" wouldn't it be asking - "how's the water?" not, "what is the water like?"
In this context, I think both would be appropriate. If I understand right, both would be a good translation of "milyen a viz" in that context.
Would something like "what kind of river is it?" or "what type of river is it?" work here? "how's the river?" is something I can't imagine asking someone.
"Milyen a vmi" (vmi = valami = something) means "What is [the thing] like?"
"Milyen vmi (ez)" is "What kind of [thing] is it/this?"
The a is an important factor here, determining whether you ask about a certain thing, or a general group of things.
It's missing the word "like" on the mobile app so it's impossible to leave the correct answer