At the risk of hijacking... (Can you even "hijack" your own thread? Much less one that has 589 replies...)
I have the benefit of having been a competitive swimmer in my youth. a lifelong distance swimmer, working with coaches, etc. The key to real swimming (vs merely "staying at the surface" swimming) is that the goal is to push water behind you. Any and every motion you make in swimming should do NOTHING but push water behind you. Doing so will propel you forward. Propelling yourself forward will keep you at the surface. When that simple idea clicks... it becomes fairly obvious and intuitive what to do... and not do... in terms of swimming technique.
Any thing that pushes water DOWN or even worse IN FRONT OF YOU will tire you out and not propel you forward. This will cause you to move slowly, feel like you're sinking, so you work harder, get more tired, move even slower and feel even MORE like you're sinking, so you try harder... It's a vicious cycle.
Check out the SwimSmooth website for
Swimming Technique: Animations & Articles From Experts Swim Smooth for some great illustrations and animations about proper technique.
If I have a student who is not a decent/capable swimmer I spend 10-15min showing them proper technique, coaching/critiquing, etc. Then I send them to the SwimSmooth website, and by the end of the class they are also better, more confident swimmers. Which of course makes for better, more confident divers. So, to a certain extent I think it is incumbent upon instructors to give some swim instruction if needed.
Here's a question, though. How many instructors get in the water and do the swim and float/tread WITH their students? I can't imagine NOT doing so. We demo every other skill before asking a student to perform it... why not swimming? And shouldn't an instructor be able to show students a "demonstration quality" stroke?