Posting because I'm a pedantic jerk.
I think modding is when you basically make an different game. When you mod in a feature something, you intend to always have that. Hacking is when you fiddle with the game just to make it work in that instance.
Assuming you will later change the reactions back to normal, you are not modding because now that you know better, you won't embark in waterless areas unless you want a challenge, and you will not make waterless areas impossible in case you DO at some point want a challenge. You would also be hacking it if you used a program (currently there are none I think) to create a block of water somewhere.
If you intend to keep playing with this reaction afterwards, even if you settle in non waterless areas (perhaps so you don't need to go outside for water) then I'd say that's a mod.
Technically though, most of the mods are hacks in the sense that they use existing infrastructure (raws) intended for a specific purpose (adding different new races, monsters, etc.) to make the program (dorffort) behave in an unintended or unexpected manner (Toady did not implement a solution to the stone problem, and probably did not intend raw modding to provide it, but meltable stone mods and the like do exactly that)