The surroundings of Tobermory, ON are spectacular. I have been diving in Tobermory several times, and every time I enjoy it. If you bring non divers, its easy for them to enjoy the boat rides, or go out by themselves and enjoy nature, while hiking, kayaking or engaging in other outdoor adventures.
Every time I have booked my dives with Divers Den. The people are friendly and helpful and accommodate my requests. Trips are well organized. Boats are comfortable and right for the job. When I come back, I will certainly book with Divers Den again.
The wrecks of Tobermory are ok. The Arabia is a great experience when you come down the bow mooring line and suddenly see the ship appear in the darkness. There can be currents and it's cold. The Niagara is a favorite of many divers because of the penetration options it offers. These would be my two top picks. Some of the other wrecks are interesting also, but many are not more than a pile of woods. Nevertheless, the surroundings, the history and the clear, cold water make for awesome dives.
Before you dive, you will need to register at the Visitor Center and purchase a diving permit. A one-day permit will be around 5$, an annual permit is 20$.