If I remember correcty and that "bug" is still there, then you can that against the sentry. The sentry doesn’t target you because you are too close to it. It targets you because you are above the tiles. Water does replace them.
The vents in the toxic gas room probably also can be replaced with water, but I’m not sure.
But the main benefit of aqua brews is equipment that interacts with water like ice and lightning effects or armor that lets you walk faster on water or the goo-potion that only heals you on water.
Though I wasn’t aware that it can actually damage fire enemies. First thought this is only about countering burning debuff.
That’s a good point and for later reference: Time @ NethackWiki
But I never really played. At most a few times, but not enough to run into something like this.
I pretty much only played Pixel Dungeon, Shattered Pixel Dungeon, Brogue and Dungeons of Dredmor.
But this is probably more an issue for speedruns than roguelikes, because I somewhat assumed that runs should be deterministic.