Uchimizu ] refers to the action of sprinkling water on the streets in the summer and has been a tradition in Japan for many many moons. The water keeps the dust low and cools off the immediate area.
The Uchimizukko Daishugo event has made this tradition more interesting by getting legions of maids to do the watering. The pink maid in this pic wearing glasses was my fave out of the lot ^o^
All these maids are from different cafes. Never been to a maid cafe before so couldn't tell you who's who. Below is a list of some of the cafes who participated in the event (in Japanese).
アキバ一丁目劇場、＠ほぉ〜むcafe、お〜ぶ、Cafe Cute M、キューティーリラックス、QUEEN DOLCE、Cos-Cha、JAMアキハバラ、SweetTrip、ハニーシープ、ぴなふぉあ、MistyHeaven、メイキュア、メイドステーション、メイフット、Merry Heart、モエシャン、癒あmaiden、ユナティコストレーガ、LittleBSD、LittlePSX、ロイヤルミルク
People who attended the event may have noticed that the organizers set up equipment to measure the temperature, wind and height of water being thrown. After water has been thrown and if there was a breeze (which there was), then the temperature would have temporarily cooled down.
I think the heat being generated by the excited men alone prevented any type of temperature cooling though.