Hi Marie,
I've never been diving in PEI, I'm in NS, my wife and I have been meaning to head up for some diving, but there's lots to see here.
I'm told most of the shore dives are quite shallow with a sandy bottom. I don't know if you like to scrounge for bottles, but the Atlantic Diver Guide by David N. Barron lists an old ferry wharf in Charlottetown Harbour as a good bottle dive. The book itself is available in any dive shop down here, and most book stores, it lists shore and boat dives for the entire maritime region.
There is also the wreck of the HMCS Assiniboine at the eastern tip of PEI. I guess it got stuck on a sand bar after being sold for scrap in '47 or so, the prop and lots of other bits and pieces are still visible in the sand it seems. Also accessible from shore.
Sorry I don't have any personal experience to give you, but Black Dolphin Diving in Charlottetown comes highly recommended to me from my LDS owner here.
blackdolphindiving.ca should be them, if not, a search should turn them up
I've heard tales of a sunken u boat off the north coast of the island, but on research it seems that nobody is really too sure where it is... if anyone here knows anything.... speak up please. Apparently you have to get a good distance offshore to find more than 50 feet of depth in most spots.
Hope this helps a bit.... let me know if I can be any more help. Dive safe everyone...
chris