[quote=Descartes]
Would desert people know what sharks look like?
[/quote]The Black Road clan gets quite a lot of work from seafaring entities, merchants and explorers mainly. One of the benefits of being able to make yourself into a human compass.
Rue has never actually seen a Shark, much less the insides of one's mouth, but she's read guidebooks and seen sketches and heard a few stories from her elders.

Though yeah, most Dune Folk don't quite have her particular education.