A five centimeter eye is a damn big one. A cachalot is hardly blind.
But there is essentially no light from the sun at those depths. So the whale finds sonar useful for the same reasons a bat does, at least. And probably much more - a whale's ear is fantastically capable, with many times the range of a human (humans hear pretty well, actually) and operating under pressure changes that would implode your sinuses.
Mammals in general hear better than other kinds of beings - better than birds, much better than fish or amphibians. They have more complex ear structures. And whales hear better than other mammals.
And they are very vocal. They sing and stuff.
