Any key to Impossible? I'm pretty sure it's the rearranging the water dudes, though I don't have the exact science down yet.
You need to rework your water dudes almost constatly. You need to know which building needs what and what it gives to plan accordingly. The game doesn't give you all the information so it is useful to write it down. Sometimes, you don't rush a building if it will give +1 on pop and then not having a project you can finish for the next 5 turns. Lastly, at the end, you can sell all your water carriers and sumps and pipes for quite a lot of cash. And you will probably have to, otherwise you won't get the money and influence in time to finish the last project. Check my post for at top screenshot :D