The map clearly shows they sailed on the side of enies lobby, its possible they were very far from it when they reached florian triangle. Anyway now that I think of it this map is very accurate: Remember what sengoku and the strawhats said in chapter 490? They're right next to the Marine HQ, and they're near the red line too. So the Marine HQ is on an island either rught before or right after the red line, not on the red line itself since when its shown you can clearly see its on an island.
Impel down, however, could very well be below the red line itself, alongside with fishman island. Sicne you know, its an underwater prison