1. make 3 cities, make one of them with tons of water and the others with no water, connect one with the water one and the other one to the one without water
2. develop all 3 of your cities untill they complain alot about water, then connect water to your water city to your non-water city, then go crazy with pipes on city 2, then connect from city 2 to city 3, go crazy with pipes again,
3. go to city 2 and buy water from city 1, then sell the water you bought to city 3, then buy and sell as much as you can,
4. Cancel deal with buying from city 1 and go to the sell menu and click the most amout of water you can sell, you now should be making money from 1,000,000$ to billions
5 (Optional). if you want to get that profit for a while pause ur game then do the buy and sell trick, then after you do that then suppy your city with tons of water, you will get the money for a longer period of time.
6. (Optional) you can connect city 2 to another city without water and do this trick for more money

Hope this helped