We don't need to add an explanation to his profile, It's clear that Mercuphobia's Water Magic is very special as both times Natsu couldn't ignite his fire, he was in Merucphobia's Water Magic, as well Gray couldn't freeze the water at all, so it's far above normal water magic and has it's own special properties, Ignia just saying that in a normal scenario Water is super effective against Fire, that's a basic law in science, water is generally more effective against fire, I don't think Natsu has a weakness to water, I mean Natsu's Fire has vaporized Stone, Sand, and Steel Many times before, these are things have a far higher vaporization temperature point than water, proving that he's far above normal water and water magic, so it's stupid to say that he has a weakness to water, Later in the same chapter, Ignia says that while "Water is effective against Fire, there is Fire that can burn even Water, just like mine", clearly he says that while Water Magic would normally be effective against Fire Magic, there is Fire Magic that can overpower water magic, which could apply to Natsu, Igneel, and Ignia, as they have fire far above normal fire, so the reason Natsu couldn't ignite his fire in Mercuphobia's Water, is that Mercuphobia is far stronger than him, not cause Natsu is weak to fire, as there is more evidence to prove that Natsu is far above water
Did you even read the chapter, Ignia literally disproves your argument
The idea is that Fire vs Water can go in either's favor depending on the power of the user, so there is no inherent weakness to water that Natsu has, as Natsu has shown to have temperatures far above vaporization of water consistently
