Sometimes teaching can be a real challenge. There are numerous water features we would like to talk about and demonstrate but it is a rare site indeed with all of these features. If teaching indoors, your task is far easier. You can certainly collect a whole bunch of pictures and video and piece together in a PowerPoint presentation. Blackboards (or more current - dry erase boards) make it easy to diagram eddies, holes, falls, etc. All instructors should prepare some form of handouts. Handouts are definitely a key sign of a professional instructor and for good reason - students have something tangible they can refer back to later on.
When outdoors, you need to be far more creative. If you have a sandy beach, you have a ready made blackboard. A small stream maybe a foot wide is a miniature class V whitewater run. Here is a great example using foamie kayakers: Whiplash Falls. It is often easier to show sieves and foot entrapments in these small streams (and certainly a great deal safer as well). You can even move rocks around to make the water feature you wish to demonstrate.