Hey Tom,
Lake Ontario is a nice looking lake, and parts of it are fine to swim in, but around the Toronto area, it's a little polluted - with actual pollutants and just debris floating around...
It's also usually pretty choppy because it's so large...
Lake Simcoe near Barrie - which is about 30-45 minutes away is pretty sweet for boarding... And, you can rent wake boats and gear from a boardshop called Tropical North...
If you're in the mood for travelling about 90 minutes, feel free to drop me an email and I'll take you out for a pull on Georgian Bay...
Take it easy!
Colin