You're right -- you have only scratched the surface! No matter which direction you look, there is more to learn. You can learn about the equipment, how it's designed and how it works, and the history of its development. You can study underwater life (marine or freshwater) and learn about the ecosystems you're watching when you dive. You can delve into diving physiology and what's known (and what's not) about decompression. You can study wrecks, where the ships came from, what they were doing, and how they sank.
And you can learn skills like hovering perfectly motionless for as long as you wish; moving over a silty bottom without disturbing it at all; kicking backwards; deploying surface marker buoys; coping with emergencies in midwater.
There is no end I can see to what you can learn about this sport, and all the process is fun (well, except for the video reviews sometimes . . . )