1) Braavos needs a ferry to the mainland rather than just a harbour on the other side
2) Volantis needs a bridge on the western/desert side. You can enter the city from across the river but trying to go from east to west just puts you back in the city
3) Dragonstone's port is on the opposite side of the island from where you actually enter/exit by sea
4) No paid option to travel by sea from major port cities between east and west sides of Westeros. The western ports only have each other as destinations, but you can still sail from Sunspear to Oldtown yourself if you have your own ships
5) The Wall doesn't stop Wildlings or caravans or anything at all. I get that it's a