I used to do this in a previous job, using different sized buckets of up to 600 gallons against forest, scrubland and grass fires.
We occasionally got asked to put out fires in wooden buildings, with some success but this method of water application would be of of only very limited use in domestic fires, if that's the sort of thing you are thinking of.
For example, most buildings on fire but worth saving still have an intact roof so the water rolls off. It can also be VERY hazardous to the aircraft and those beneath, bearing in mind that 600 gallons of water weighs 6000lbs plus the bucket.
I have the Gazelle on my licence and used to instruct on it; although I never flew a version with a hook I don't honestly think it would be powerful enough to lift enough water to do anything much more than to make a token effort.