I think a lot of new divers have a bit of a misunderstanding of the importance of depth. I think their reasoning works along the lines that they have probably seen more on their early shallow dives than they did snorkelling and so if they do deeper, more advanced dives there will be even more to see. Whilst deeper water can and does bring in different species and dives in the 30metre range at places like Ras Mohammed NMP (Egypt) or Protea Banks (RSA) attract big numbers of large pelagic fish this does not mean all dives need to be deep to be successful. Different depths attract different species and many of the world's great muck and critter dives are less than 10metres deep. Even divers who are familiar with these dives often ignore the extreme shallows where the water is less than waist high. Whenever I shoredive however I always make sure I dive the way back in until I can literally go no further and have to stand up - its amazing what you can find.
Recently I've been spending time snorkelling a shallow salt flat. Whilst most of my finds have been small like this one I've been seeing some bigger stuff too.