Gene,
Tell us more about what you need -- one idea would be to go back to the book/text you used during your certification course and gather information from there. It would be a useful guide -- you can go through it and decide which topics you want to talk about -- equipment, the various "laws" (Boyle's Law, Dalton's Law, etc.).
One idea would be to use "adaptation" as your theme. I recently did a scuba presentation to my son's elementary school class where I put on all my gear and then entered the classroom to give a little presentation. I focused on adaptation -- how humans have to adapt to living for short periods underwater and how the various pieces of gear we use helps us to do this effectively.
Let us know if you have more questions --- good luck!
-Chris