Placing a water pump (using filtration pumps) directly next to a sewage treatment plant allows for infinite water. The sewage treatment plant adds back in to the ground water table at that spot. The filtration pumps ensure your water won't get polluted from the ground pollution that will occur.
Using this trick, you can pick 1 of the 4 clustered cities in your tradeable region to use this trick over and over. This will provide a massie amount of water that can be purchased. You can then buy from that massive source while building the other 3 cities. This will save needing to place a water source and sewage treatment plant. This can be done for any utility/service.