No matter how much I think about it, the only meaning of Luffy's message that makes a bit sense is that he's saying they have to go to Water 7.
- He makes one round around Marineford as a symbol for a "burial at sea". In the eyes of the marines and the normal people this would mean he's showing respect to the ones who died at the Summit. However for the SH-crew this could be understood as a reference to the Going Merry, who had a burial at sea.
- He rings the Ox Bell 16 times. For the marines and the other people this would mean the end and start of a new age, but for the SH-crew this could mean 1+6 = 7 => Water 7
- He bows his head, which I think he also did when the Going Merry went down, so this could be again a reference to Merry that sunk close to W7.
It makes sense that he would use something like the death of Merry, which only the SH-crew can understand, to create a message. The problem with this is that Brook never has known Merry. Normally they should have told him about it, however then the question remains if it would be enough?
In the case to make sure that everyone would understand it, it should have something to do that happened after Brook. Problem here is that nothing much really happened.
- As Rayleigh said the key is something completely not Luffy, but what does that mean? Well Luffy always goes forward to the adventure, so saying that they need to retreat is really not like Luffy.
Though I find this all way to superficial and I could be just seeing things. The only other explanation is that they refer to the SA, but what use would it be to send a message then as they were already on their way to that place?
