We were at Tobermory in June, and on the deck of the Niagara II in 78 feet of water, the temp was 34 degrees. I mentioned this later that day to the gal who runs Divers Den and she shook her head and said it NEVER gets much warmer there than that. That wreck, and the Caroline Rose are really out in the open and anytime there is any wave action, the water mixes up and stays cold. There were 9 of us on the boat, and every one of us was in a dry suit.
Now the dives in Big Tub Harbor are quite a bit shallower and the water gets warmer because it's mostly protected.
We dove with G&S and they are a first class operation. I understand that Divers Den is very good too. Both shops are complete and in good shape and we didn't have any problems with the air from G&S, even in the very cold water.
On Sunday, when we were going out to the Niagara, I saw a boat from Divers Den load up with folks in wet suits, so it certainly seems 'do-able', especially in the warmer part of the summer...
Hope this is some help....