In the Great Lakes area, it would violate a provincial-state agreement.
There's an interesting history here that you might want to look into. During the free trade negotiations, the federal government of the day introduced a bill called the Canada Water Preservation Act, which would have prohibited water export from Canada. There was a free trade election; there was a free trade agreement. Afterwards, you'll note that proposed law didn't come back to the House. If you were to get a serious bunch of lawyers together, I suspect you'd find out that the reason it didn't come back was because it couldn't come back.
We probably couldn't pass of a law of that sort today without violating international trade agreements. That's probably why the Canadian government has a policy based on river basins, rather than on countries. It's likely that we would run into problems if we tried to absolutely prohibit export from a country under the current trade arrangements. We can do it between river basins for conservation and environmental reasons, but to do it between countries would be questionable today.