Uh, I strongly doubt that Haki makes someone a human seastone.
Blackbeard's DF power is more akin to something like that. Besides, if that were the case, we wouldn't have seen Kizaru dueling Rayleigh with a light saber, would we?
Anyway, I made the comparison a few times in the past, but I don't think I've really elaborated on it: I see Haki as being very similar to Hamon (or the ripple) from the earlier parts of Jojo's Bizarre Adventure. See, the way Hamon works is that a specific breathing rhythm creates a ripple-like wave of energy. This way, it was possible for a person to transfer their own energy through other sources like weapons and were also able to utilize the power of sunlight (for instance, Will Zeppeli was actually able to transfer his ripple energy through a
glass of wine). When we first learned that the Kuja amazons could imbue Haki energy through their arrows, Hamon was the first thing that came to my mind since it was used in a somewhat similar fashion.
Now, whether or not Haki requires a specific breath pattern to be used, I don't know. But I DO think that it's related to breathing, along with willpower. Think of when Zoro fought Daz Bones at Alabasta. He was near death when he was suddenly able to hear the "breath" of the rocks around him. Then he was able to hear the "breath" of Daz's steel body, and was able to cut him. Now, in Jojo it was explained that ripple energy can be used to interrupt another ripple. You could say that Zoro's "ripple" interrupted Daz's ripple on the
very moment he cut him. That's why I think that Haki doesn't truly "cancel out" a DF power, but it DOES "interrupt" it in a way that renders them defenseless, even if it's for a split second. Perhaps being half dead made Zoro breath in a way that he was able to sense the "breath" of all his surroundings during that moment, I dunno. But those are just my thoughts on the subject for now......