They literally did used to have water-themed spells, I think the longest-standing one was Fluid Aura, it's just that they were gradually pruned off of the toolkit because they didn't really make gameplay sense. Back in 1.0 Conjurers had access to every single element because that was their 'thing', including Water (Thaumaturges instead got light and darkness magic), but come ARR they realized a heavy focus on elemental damage didn't really work for the game they were making. 'Conjurers don't use water magic because something something Sixth Calamity' is just pure headcanon, and it's not even headcanon that has a long memory for exactly that reason.

As for why no NPC conjurers use water spells in-game, outside of the mechanical reason? Well... they've mastered magically hucking rocks, controlling the wind and healing wounds; practically speaking, what is water magic going to do for them that they can't already cover?