Yes, the sea bit of the journey seems to be the most problematic. CraigR's link to Burl's essay at http://www.britarch.ac.uk/ba/ba45/ba45int.html points that up.
Let's face it, given enough time and manpower practically anything can be moved anywhere overland - the same cannot be said about sea crossings (though I hasten to add that I'm not saying they didn't or couldn't do it :-)