At present, it looks like all or almost all of the seabound monsters are those that just happened to wander into the sea. They will most likely wander back to approximately their point of origin (so most of them will land back in rogue lands).
The whole point of allowing monsters to go by sea, though, is so that it will be not just border realms like Madina that get hit with the monsters, but all the realms that border the inner sea.
