I noticed this recently, whilst in this instance there's several columns that support walkways etc. The base of these columns are surrounded by water, but you can stand right next to the column and fire without any problems.
The atta mobs in the instance don't like water, hence you're free to kill as you please, you will have to take out the Workers, Servants and Taskmasters who have ranged attacks but any of the others that you lure to a watering hole will be completely at your mercy with you protected by your moat of water. They'll wander around the water but never enter, I'm not sure if this was intentional or not.
Aim and fire at will
