@Sammsy:
Luffy had plenty of time to reach marineford with normal ship, just that going there was more risky than impel down so he went there instead.
No he didn't. He got to Impel Down with a day to spare. Marineford is half an hour away, so they're basically in the same place. Luffy NEEDED the shortcut of the currents to get there in four days, and not more than a week, and he was actually in the general vicinity.
http://www.onemanga.com/One_Piece/522/10/
But anyway besidse Luffy, we have 47 new world ships + those of whitebeard and his crwe that who knows where they were all scattered in the new world.
I assumed they used transponder snails and vivre cards to get the word out and met up on the way, rather than all gather, and THEN move. And we have no idea where WB was generally situated, particularly since he WASN'T interested in One Piece, there's no reason to assume he was at the end of the grand line. (insert speculation about Mihawk meeting Shanks mere days after first meeting Luffy suggesting Shanks was in the old world for a while.)
But they ALL managed to get back to the old world to marineford in time, which includes getting all their ships coated. And hmm it takes rayleigh 3 days to coat a ship. not saying you can't do it faster, but its not just so simple as reach red line bow a bubble in seconds and head on through.
We still don't know whats involved in coating. It might be a one time process that lasts forever, or that can be done while travelling. In either case it wouldn't factor at all for ships that have already been to the new world and had to go through fishman island at some point.
The fact that Sengoku seemed so surprised by the submerged ships and hadn't really planned for them during the war indicates that there's a lot we don't know about the process yet.