It might help to know the area you are in or are planning on visiting, maybe someone can make a recommendation for your area. I take my dogs swimming all the time at a couple of spots near me. Not public beaches per se, but just remote areas on a couple of rivers though one of them is a public access point to the water where people will occasionally put a canoe or kayak in the water. Also a river flows right through my town and a lot of people take their dogs to the beaches there at Rotary Park.
I'm not sure how many of the provincial parks do have dog beaches, the only park I go to is Algonquin (in the winter for dogsledding so no swimming then
) I'm pretty sure on the Algonquin website I read that dogs are not welcome on the beaches. You should be able to look up info on the parks if you google Ontario Provincial Parks.