Ethel - The main reason is that (though it isn't clear on the TV) they are driving over a series of lakes and 'land bridges' in between them. This would mean you would need a ship for each lake, a dock and facilities at either end, and a truck on each land bridge. Also, many of the lakes in Canada are isolated...that is they have no river in or out, but are purely melt water in a 'hollow', so you can't sail from one lake to the other.