I will give u the format then u can frame the letter yourself.
Write the adress , date part.Now the subject should be-Requesting immediate action by the concerned authorites on the matter of illegal water connection.Now for the content divide the letter into 4 paragraphs. In the first paragraph u need to describe the subject of ur letter. Start like this, I would like to draw the attention of the concerned authorities towards the increasing number of cases of illegal water connection... Now in the second paragraph write why and what action u want to be taken to bring the matter in control.
In third write wat are the priblems being faced by those who pay for the connection and in the fourth paragraph request for immediate action to be taken. Now write ur name along with thanking u and urs faithfully.