@Katzztar:
Frankly, going into the Grand Line should be avoided as it would waste time going to island to island unless they have an Eternal Pose to specific island. But then they would need another eternal log pose to reach the next island with a hostipal/doctor or they would have to travel the normal way with a regular log pose = wait for it to set and point to next island on that path.
That's one thing that has been buzzing me since the beginning of the Dressrosa arc. Up until Punk Hazard, Grand Line islands were remote isolated places that barely allowed any travelling, trade even less. One had to rely on either a log pose (a rare item if I remember correctly the early chapters), an eternal pose (even rarer), or the Puffing Tom (quite unique). Or be a marine and take the safer route via Calm Belt (still quite specific).
Now compare Dressrosa. Suddenly, champions from all over the New World visit the island for a day at the coliseum. Doflamingo built a whole empire on smuggling weapons and Smiles. King Rikku used to travel the sea to help the neighboring countries in their wars…
Long story short, sailing the first half of Grand Line was a thing of adventurers, whereas it seems quite commonplace in the New World. The likeliest explanation is Vivre Cards, or maybe that sea is so nasty that only navigators who are awesome beyond imagination ever make it there. Or they mass-produce log poses on some island. Whichever, this may change much compared to the adventuring cycle the Straw Hats went through in Paradise.
On a completely unrelated side note, I'd say even Grand Line can have maps (we've seen a couple of them, actually, although they all focused on a single island). Just because normal compasses lose the north doesn't mean navigators venture there completely blind or suddenly forget to take notes of where they went. (Not that this would change anything about the sea Cora and Law are supposedly sailing)