> Intriguingly, the route lets you see a bit of France as well, rather than directing you up the Channel directly to London.

Well, no matter what points you choose in North America and Europe, it will tell you to swim from the Long Wharf in Boston to that place in France (or vice versa). I guess all other routes are quite polluted. :)
However, it has no idea how to get from South America to, say Africa. Or from San Francisco to Tokyo.