Let me know if you head up to Tobermory. I go there at least a handful of times a year or more and it is a wonderful spot.
It helps to have a guide. A Freediver friend owns a bunch of cabins up there "Little Cove Adventures" and is ideal for freedivers looking for a place to stay, have a campfire, etc.... But gives you a comfortable bed and running water for your neti pot!
The underwater rock formations are truly impressive. There's one spot I love where you swim down 20m and you're at the bottom, looking up at a huge 50-60' cube of rock that's bigger than a house and you feel so insignificant and tiny, yet it's so quiet and peaceful.
There aren't a ton of fish, although we've seen the odd large Pike, a few larger whitefish, schools of small trout and if you know where to look... You can find bass holing up in the rocks.
In Sept supposedly, you can swim in the local harbour 15m deep max in swarms of large salmon that are migrating to spawn.
The 2 species you see the most are both invasive: Zebra mussels and the round goby. The gobys have to number in the billions. 2-4" little bottom-hopping fish.
There is some good shore diving around rock formations and a few shore-accessible wrecks. There are a few good shore-accessible spots for line-diving as well.
A boat will get you to some of the better / deeper / more intact wrecks.
The caves / swim-throughs in Cyprus lake park (A.K.A. The Grotto) are impressive, yet very attainable and quite safe.
YouTube has some videos of Freedive Toronto in Tobermory. It'll give you some idea.
May water temp is between 2* and 7*!
It gets warm (on the surface) in Aug and is good in Sept as well.
I dive it May to Nov though.
It's some of the best freshwater diving anywhere.