From an editorial in the Toronto Star:
As aquifers and rivers in the United States run dry, there seems little doubt that the Americans will be turning their thirsty eyes north toward our seemingly plentiful water supply. Indeed, no less an authority than former Alberta premier Peter Lougheed has predicted: "The United States will be coming after our water in three to five years."
But isn't our water protected by federal-provincial agreements and legislation, such as Bill C-6, which amended the International Waters Treaty Act in 2000 to ban water exports from the Canadian side of the Great Lakes and other regional basins?
Maybe not. There is a concern that such protections would have no real force against World Trade Organization rules, which prohibit the use of export bans or limitations on any product that a trade partner has contracted to buy – especially when these WTO rules are considered in combination with the provisions of the North American Free Trade Agreement (NAFTA).
Summing up this concern in a report this week, the Polaris Institute declared: "It is not at all clear that either Ottawa or the provinces are in a position to deal with a challenge coming from Washington to turn on the taps for Canadian bulk water exports to the U.S."
Here's the Polaris Institute report mentioned in the article.
I'm not sure that there is really anything to fear from WTO rules on this. A water export ban or limitation would clearly violate GATT Article XI:1, but I would think that it would not be too hard to justify it under Articles XX(b) or XX(g).
With regard to the NAFTA, the report seems to focus on the Chapter 11 investment provisions. I wouldn't have thought Chapter 11 would prohibit such regulations, but maybe the report has a point here.