In addition, I don't think monsters have ever crossed sea channels/sea regions, so there's little reason to believe that's changed.
Monsters do, indeed, cross seas, using the old sea routes. We saw this quite regularly in Astrum in the early days, with monsters routinely going back and forth between Mimer and Libidizedd, and Mimer and that-connecting-city-in-D'Hara. I can't see any reason why a character would believe that monsters can't cross the oceans on sea routes, unless it's some kind of RP thing. It is probably rare on Dwilight, though, as sea routes terminate in fortified cities. You don't often find situation where monster groups can beat the militia, and then survive to travel on to the next region.
