It isnt just to stop you from damaging the environment. But it is a considerable risk to damaging yourself and your equipment.
First air consumption. Most beginner divers eat up air like crazy. How long does your tank last you? Most people start of lasting about 30-40 minutes on a tank. Expect the extra drag from pusing a big camera to reduce that by up to 40%. Thus your air may only last 20 minutes. If you do 5 minutes down and 5 minutes up you may only have 10 minutes of picture/dive time. With everything new and exciting the chance of you exceding your 10 minutes and running out of air is very high.
Like everything else, air consumption is different for everyone but I would recomend you wait untill a tank lasts at least an hour before you consider bringing the camera.
Next is you must learn to do all the skills you learned in your open water class with one less hand. For example. The currnet is somewhat strong so you must hold on to the anchor line during descent. How do you plan to clear your ears, hold your camera, and hold the anchor line at the same time? 3 hands would be very usefull. If you let go of the anchor line for even a second you will be swept off to sea.
Do you think you would be able to put your mask on with one hand?
And then finding your equipment. It is very important in diving in general to be able to find each piece of equipment very quickly without looking because it is impossible to see your chest where all your gear lives (BC controler, console, weightbelt, octopus, BC clips, BC pockets, ect). Now adding a big camera, doubbles the ammount of gear you must keep track of and now you only have one hand to find it all.
I do like kens suggestion that you make sure you are listening to the Dive Briefing. I am often use the dive breefing time to make sure the camera is together right and working as it should. This is not a good idea as dive briefings are quite important to help you know what to exped down there.
I have met divers with 10 dives who were much more in control than divers with 50 so of course how long it takes you to get the diving part of things under control is something you will have to find out for yourself. You may allready be there or it could take you more than 50 dives to figure it out.
It takes most people years to get up to 50 dives. Thus my suggestion to get a small camera that you can handle now rather than no camera. But if you are willing/able to wait untill you have the diving half rock solid I'm sure you will love diving with your 10D. I hope you can see we arent trying to be condesending to you as a new diver but are trying to help you become a good diver and good underwater photograper. But you must do it in that order.
RE: Fast shooting above water != fast shooting underwater.(!= means not equal) Have you ever tryed to take a picture of a rabbit, Squirrel or similar sized animal with a 20mm lens before? Thats what your going to be trying to do under water. If you want a fish to show up in your pictures you are going to have to be at most 2 feet away from it. You dont do anything fast while diving. Hopefully thats something your instructor stressed. The same holds for shooting. If you want a picture of a fish you are going to have to slowly stalk it or it will swim off.
RE: Dive certifications.
No you do not have to stick with PADI. Dive instructors like money and will not turn down a paying customer even if their last course was from a different company. You will find that different companys use different tables and have different guidelines so it will be more work on your part to learn 2 sets of guidelines but that is the only issue.
I dont know anything about dive conservaton programs.