"He swims like fish in the water."
Translation:Úszik mint hal a vízben.
Okay, now I'm confused... when do you use and when do you not use olyan? Does olyan get omitted where "like" functions as an adverb?
You've probably figured this out already, but still.
Olyan & ilyen work as adjectives, while úgy & így work as adverbs. Here "like a fish" describes the way he swims, so it has an adverbial meaning, there you can use így, but not olyan. Therefore another possible translation is:
Úgy úszik, mint hal a vízben.