Why wasn't water used to defeat the Borg?
The Borg are able to adapt very quickly, I think you only get one or two shots with a phaser before they adapt.
So, instead of phasers, why not use water?
Water and electricity don't mix, the Borg can't adapt to it, so water would always kill them.
Imagine if water guns and/or water balloons were used on the Borg? That would be cool and also kind of funny to watch.