Shouldn't be any need to flush since it's fresh water. But, if you're worried about any debris that may have gotten into the cooling system you could flush it just for piece of mind.
I was out in the Toronto Harbour a few weeks ago. Yes, you're supposed to have a Harbour License but I didn't see any sineage enforcing this, and I launched right in the harbour (Port Credit). Like others, I'll just plead ignorant...