Are you looking for shore dives or boat? If you're looking at Lake Simcoe, you've probably discovered the JC Morrison in Barrie (one of my favourites when visibility is good). Jackson's Point is shallow but the old marine railway is interesting and last dive there were hundreds of small bass and sunfish hovering with excellent vis. I'm told there are remains of an old lifeboat, but we were unable to locate them.
If you've access to a boat, there are some relatively shallow wrecks around Christian and Hope Island. Directions can be found in Chris Kohl's book on Great Lake Shipwrecks (if you can find a copy, currently out of print). I haven't dove on them yet, but plan to next summer.
We've been exploring around the Meaford area, so far nothing of great interest, but good vis most of the time.