There is lots of Ontario diving.
The positives, its cheap enough and close to home, Tobermory is pretty interesting, the great lakes hold hundreds of wrecks, St lawrence has some good sites. There are a few flooded quarries that are now dive sites. You can meet some fun people so if you do a dive holiday you can find some people to travel with. Most dive shops have some group of regulars that have a 'club'. Diving with a small group that has experience in the area you want to see stops a lot of wasted time and is just safer.
The negatives, Ontario water is COLD. There are just shades of grey, you dont ever get cool colors like you see in the southern salt water. You need decent gear to stay comfortable, there is no diving in a light weight shorty wetsuit.
If you get sort of serious , look for a dive club, they can be a great source for training, deals on used gear, travel advice, local trips. Like any groups you'll find everybody from fun guys to Jacque Cousteau reborn, but that sorts itself out.