I have the answer. Simple than you all thought and probably any of you will like it.
Oda is inconsistent with his story and the elements he introduces. Period.
What is the point of Kairoseki and Sea Water again? Because Luffy is using everything, haki and DF powers while in touch with any of them. Okay now I'm not sure if he used DF powers while in touch with Kairoseki but he did underwater..