I had some thoughts on this, but they never made it to the final setting – don't remember why really. The main point from a gaming perspective is that we don't want people to travel to Alderland since it's not there yet obviously. Also there was hesitation on letting people travel from Alderland to Ravenland since we don't want other then anecdotal knowledge of Alderland in Ravenland. Otherwise I think it would have been neat to have criminals lowered into Ravenland, never to be allowed to return (much like in the movie ”Escape from New York”). My first map draft had a mine camp close to the Iron Lock, where newcomers were welcomed (as workforce / slaves / goods) and were given a first introduction to Ravenland.
Concerning preventing boating, there was talk about godly sea monsters or a giant maelstrom in the water, combined with insufficient knowledge of building ships able to leave the coastline. As there probably will be boating in an upcoming expansion, this will not work.
So you can either allow sailing to Ravenland or stop it in some other way. My suggestion is a device I used in a world I drew in the 80-ies and 90-ies, where the sea surface is not necessarily flat in all places. Imagine a permanent immobile wave hemming in the Ravenland coast – a water ridge. You cant sail upwater since your boat will simply slide back from the attempt to cross it. I think that idea is pretty elegant and not too hard to accept.