Water drops from altitude are complicated by atmospheric resistance and wind -- too much water misses the target from wind or dispersal and some vaporizes. So why not drop ice instead? It doesn't have to be blocks. Flake ice might do nicely. Additionally , the two phase transitions (from ice to water to steam) would seem to give you more cooling. Ice can be transported/airlifted to the site, or can be made on site. Commercial flake ice machines can produce 30+ tons a day, and are relatively simple machines that can use salt water -- or water mixed with boric acid or whatever else you want to mix in. Fish processing (and I'd imagine whaling) ships often have such large ice machines because you need a lot of ice to keep fish from going bad. So park one or two of those ships right off the coast and have a helo fly out to pick up loads of flake or other types of ice, right off the deck. The ship doesn't need outside electrical power, there is plenty of seawater to use.
Although there may not be "off the shelf" commercial gear, you could also use pnuematically (i.e. air pressure) propelled flake ice from a firehose or cannon. Ice propelled at the same velocity as water should travel further, disperse less, cool more, etc. (it is the phase tranistion of melting, and not the temperature per se, that most effects cooling IIRC). You could also try and "split the baby" by injecting flake ice into the water stream of a more conventional fire/water system - in effect making it a slush stream. Might still be significantly better than just water.
(and no, I'm not selling ice machines....) :-)