"He swims like fish in the water."
Translation:Úszik mint hal a vízben.
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.