Transport Canada recognizes seatime on US vessels, but they do not recognized any US credentials. Transport Canada will not issue Canadian credentials to anyone who does not have the authority to work in Canada. Canada's licensing system is based upon the British system. Its much more difficult to get a license in Canada, than in the US. http://www.tc.gc.ca/eng/marine-menu.htm
Maritime jobs in Canada are mostly union, and there are a lot fewer vessels and jobs in Canada. I only know two Americans that have managed to get seagoing jobs on Canadian vessels, but I know quite a few Canadians working on US vessels. A Canadian with work authority in the US can get a US MMC, but cannot get a US license unless he is a dual citizen.
It seems that quite a few Canadians are dual citizens of both countries, but very few Americans are dual citizens. I haven't met many Canadians that do not have relatives in the US, but there are not that many of us with relatives in Canada. Many Canadians have worked at one time or another in the US, but not many Americans have worked in Canada. Until 20 years ago, there weren't many jobs in Canada, and many Canadians came to the US legally or illegally to work. They still do.
You cannot even visit Canada (even as crew on a US flag vessel) if you have ANY kind of a criminal conviction (i.e., drunk driving, reckless driving, simple assault, domestic violence, etc. (a lot of little things that are no big deal here)).
In theory, its not too difficult for Americans with needed skills, or a lot of money, and no criminal record, to get a work permit in Canada. In reality, you have to really want it, and work hard to get it. Getting a Canadian maritime license and work on a Canadian vessel is extremely difficult at best.