Professors Lake is located in Brampton, close to the intersection of Bovaird Dr. E. and Bramalea Rd. I first went there in 2000 out of curiosity, a couple of years after the piranha story made the news (in 1998).
When I got to Professors Lake for the first time, I was fascinated to find a lake with clean, swimmable water in a wholly urban environment: what a rare treat and treasure!
The lake is a former quarry, which explains the water quality. As far as I know, there are no stormwater outfalls fouling the lake.
I swam there for the first time in 2003. The sandy beach is nicely graded, and water clarity is very good—especially when there are fewer people using the beach. Otherwise, the water can become turbid. The lake is reportedly about 50 feet deep at most.
A floating diving dock is sometimes put out into the deeper part of the lake, which is about as interesting and uncommon a thing to see as is an urban lake managed for swimming.
To the local authority's credit, there is also a seasonal boat rental at Professors Lake (unlike in High Park, where it was taken away 30 years ago and never rebuilt). The boat rental docks could, however, be relocated to a shallower part of the lake.
I'm happy that Professors Lake has thus far not been subjected to the same "naturalization" treatment that befalls many other water bodies, where the local authority cancels recreational activities and opts for no public access to the water, perhaps installing splash pads or a pool instead. Such places are simply micromanaged, and keep people from interacting with the resource.