As you know, Sanji has a move called Diable Jamble, where he somehow manages to generate fire on his leg by rotating with great speed.
We have seen this move first when he was fighting CP9.
According to science, fire can be generated by friction, but it should be spontaneous. This explains as to how he was able to generate while he was fighting CP9 at Enies Lobby.
But after the timeskip, when the Straw Hat Pirates were on their way to Fishman Island, they encountered a Kraken. How did Sanji generate fire in water?
In water, there is no rough material via which, fire can be generated.
And even if it is somehow achieved, how can the fire not get extinguished by the water!?