Is There a Reason Why Conjruers and White Mages Don't Use Water Spells?

[Rongway]

'Conjurers don't use water magic because something something Sixth Calamity' is just pure headcanon

Not conjurers, but specifically the Amdapori white mages; and it's not that "they don't use water magic because the Sixth Calamity" but rather "the Sixth Calamity was a flood (of water) because they didn't use much water magic". You've got the cause-effect backwards.

We know from the Red Mage quests that what separates red magic from its white and black magic origins is that red magic uses just the caster's personal aether, which makes it more environmentally friendly than the white and black magicks of Amdapor and Mhach which used ambient aether. From the EEvol1 we also know that the white and black mages fought for so long that in the end there was so little of anything but water aether left in Eorzea that it brought on a Calamity of Water.

Three centuries of ceaseless assault on the land had taken its toll, the drain of energies used to propagate war sending the realm's elemental balance askew ... A Calamity was upon them, and soon the waters would rise to purge the land of those who had ravaged her so.

We are meant to infer from this that the black and white mages significantly drained the land of all the elements except water. Therefore, the Amdapori white mages didn't use anywhere near as much water magic as they did earth or wind.