Probably because that graphic took a really long time for whoever designed it, and there was no harm in replacing the old graphics/tiles piecemeal. I could be wrong though. It's a very well done graphic, I might add.

I wouldn't mind a level where people are actually sailing and the ship has scheduled shops at ports like pre-NPC Server Classic. Maybe that's the plan? Maybe they want the ships to sail on the actual overworld instead? Maybe neither?