NP says that the key to capturing great footage and pictures is to be very patient and to know and understand the species that you are filming. If you know their behavior you will know where to look and what to expect. Many divers invest in fish identification books/slates that will help identify the various species you might encounter.
To get started in underwater photography, it might be a good idea to take an underwater photography class. You will learn many skills that will be useful in capturing excellent shots. The other class that might be useful is to take a buoyancy control class. This will help you develop pinpoint buoyancy control.
A good camera, housing, good lights and patience will make a huge difference to your enjoyment of the marine world.