Any top tips for how to approach learning this? I totally understand what you're saying but aside from seeing 'stuff' (ok, not quite so crude) I still feel very much ignorant of my environment. Books? Talking to people? Just curious.
It depends on your environment, but in general the thing that has helped me the most is to just ask questions. If you see something on a dive, and you are puzzled, try to find out what it was. Ask the people you are diving with, the captain, or the DM. They know the answer most of the time.
If you're a reef diver, you're not going to learn all the fish at once. Whenever you see an interesting fish, etc. just remember it, and find out what it is. Make notes. Look it up later in a Fish Book. Learn how it behaves. Try to be able to consistently ID it, and find it in different stages of development. Watch it, and see how it behaves. See what environment it likes to live in, and what other critters you usually see around it. In time you will start to make associations, like "Hey, there's a lot of sand stirred up over there. Let's investigate-Oh, it's a stingray digging out lunch." or "hey, a bunch of crab shells, I wonder if an octopus nest is close by."
If you like wrecks, just learn all you can about the wreck by reading up ahead of time. In time you learn things just from listening to the boat talk. Find an interesting feature, and just ask later on what it is. I like Great Lakes wrecks, so I read up on Great Lakes History. Learning about the different locks, etc. gives a good background on why you see so many ships of a certain era that standard dimensions, etc. (to fit through the locks).
Then there is also the comfort part. I can't feel at home unless I've got my buoyancy, etc down, and I feel relaxed. That just takes practice and time.
HTH,
Tom