This is something i wrote as a comment in someones post which i disagreed strongly. And hopefully you will agree with my thought process.
海楼石 Kairōseki which literally means 海Sea 楼 tower 石Stone. Translated as sea stone or ocean stone Another meaningful translation would probably be sea floor stone which makes sense 楼 is floor in original Chinese. We should get the idea that Kairōseki is something which is formed in or obtained from sea/ocean floor.
Island Clouds are a special type of cloud. They are fluffy, but dense enough to carry all kinds of things. They form islands in the sky seas.
Island Clouds, as well as Sea Clouds, are created when a Molecule in Seastone called "pyrobloin" is ejected into the air by volcanic eruptions and comes in contact with water vapors.The density of the water vapors determines whether Sea Cloud or Island Cloud will be created. Both these types of clouds are not normally able to survive on the Blue Sea.
Pyrobloin パイロブロイン Pairoburoin is a fictional word. Its made up two words pyro and bloin.
well that is what the VIz translated it as pyrobloin, doesnt mean spelling is correct.
pyro is as we know fire and bloin is nothing but there is a word called Blouin derived from old french word blou ‘blue’. probably meaning fire and blue sea. As we know pyrobloin is ejected into air by volcanic eruptions from the sea we can expect it being named because of fire in sea causing it to be released and separate it from other impurities.
They then mix with water vapors.
we know density of water vapors is higher in sea clouds since its more liquid compared to island clouds.
water density is 997 kg/m³, water vapor density is 0.554kg/m³.
And as expected Sea clouds have slightly less density than sea water.
From this info again the only thing which effects DF user was the sea cloud which has water which should not really be sea water since its the H20 from water vapor which is sterile.
Now if we add sterile water + pyroblouin which also are present in sea stone and the fact that DF users are effected by it then we can conclude it is most probably because of pyroblouin that DF users are ineffective in sea.
Sea stones also contains them as already confirmed and they are hard as diamond and i guess they are formed at sea bed i.e. on the floor. And also salts contain it obviously.
In real life the most dominant ones in sea water are Cl− and Na+ ions which we obtain as sodium chloride. So i guess the most dominant might be pyroblouin. Not necessary but highly possible.
EDIT - About table salt not effective against DF users. tldr quantity is very less in salt used for it to effect DF users and we use very less salt in food too.
edit - most probably the pryoblouin like sodium and chloride ion is abundant in sea being the reason as to why its dangerous everywhere, there is no place in sea where is no pyroblouin present and if you make salt from sea that would mean it has pyrobloun like sodium chloride...think that either sodium or chloride is pyroblouin it would make sense then. And yes that was zombies r weak to salt but he gave a reason. He cant randomly do that without any reason or else it would be a plot hole and brook said it works against zombies instantly it might work on DF users too but is negligibly in its effect so maybe not much pyrobloiun. assuming its not hole in writing.
So we can think the salt is obvioulsy NaCl in one piece too but pyroblouin is also present like other ions like silicate maybe.
Brook clearly says the salt has power of sea so if we have to make an excuse salt made for eating is mostly contains NaCl like in real life. And also the table salt we use also is not pure NaCl, it atleast contains Iodine so its a homogeneous mixture maybe. And pyroblouin used is like i said a very scarce one present in sea.
no need to take these numbers seriously though. So yeah if we want to make an excuse we should say the power in salt is not strong enough because they bring salt from kitchen in TB and it works. That would be a lot hole unless we say its not much effective against them since concetration of pyroblouin is very low. I mean how do they get drinking water out in sea. They can filter it maybe that process removes pyroblouin for drinking since fresh water doesnt has pyroblouin. Its similar to how in alabasta they were being flooded but it doesnt mean they were drowning like in sea but still were effected just to a very small degree. Two reasons could be the surface area in contact and concentration of pyroblouin is very sparse in ocean water and having less water will make it even less in quantity. If we make salt would mean we are collecting every ion and would make pyroblouin to be collected to which is one of the lowest composition in the salt and we use very less salt compared to the food, so its probably ineffective, like soaking someone with sea water doesnt make them faint etc. Its probably not a hole since it can be explained with some or other reason.
Remember for Oars they used large quantities of salt.
Did we miss something about ussop. Were his marbles made of sea stones. Wonder if its going to be something very simple which he always played with unknowingly.
One thing which i wonder is that whether oda will scientifically explain as to why Devil fruit users are weak to this. Or it will be kept as a supernatural mystery. Like davy jones being cursed by sea since it makes things more interesting to pirate like stories.
Theory What Causes Devil Fruit users to become weak in Sea.
Will give the cause for DF users to lose power in sea.