Ans to this riddle is
Because there are river "banks" along the river and fishes can rob them because there is no such thing as a fishy policefish catching theif fishes red-handed and sending them to jail.
But... if you seen a fishy policefish before do tell me!!
And while we're on the topic of sea jokes............
Said the sea urchin: Spare a penny, mister?
Said the goldfish: What? Do you think I'm made of money?
School is practice for the future. Practice makes perfect. But - nobody's perfect, so why practice?