[I take no credit for this]
1. make 3 cities, 1 with tons of water and 2 without, like this
_________________________
|-------------|-------------|
|----Water---|----No water-|
|-------------|-------------|
_________________________
....................|-------------|
....................|----No water-|
....................|-------------|
2. Connect water pipes from the city with the water to the city right by the water city.
3. Connect water pipes from the city that had the water connected to and make sure its not the one with the tons of water, to the one right below that one. [Make sure theres pipes going everywhere in the second and third city.
4. Let the three cities develop untill the second and third city are complaining about water.
5. go into the second city
6. now go to your budget screen, like the dollar sign at the bottom and click on the money thing that appears,
7. click on neighborhood deals
8. buy water from the city with lots of water
9. click accept and go back to the screen
10. now sell the water, buy, sell, buy, sell untill you cant buy and sell anymore, not stop buying water and sell the water at the highest amount and then click accept, they money you can get monthly for like 2-4 months is from 500,000$ to 5,000,000$
11. Repeat steps 7-10 untill have as much money as desired. Enjoy!