I usually don't sell water because it isn't necessary and when I first played the game it always seemed a head ache. However, I am trying a difficult terrain (deep rivers with high landscape) and need extra cash.
Set up separate water pumps with a separate wire to power.
However, if something happens to the City water (earth quake, tornado, or my stupidity), the deals off and I get hit with a penalty. Not that big of a problem as the City is making enough now. But is there anyway to stop this. Or is it hardwired in the game that problems in the City cause problems with the water deal even if the water going to the neighboring City isn't effected.
Thanks

Water rules as best I can figure out.
I had a separate electrical source to separate pumps to the City to the North.
To keep that deal, I had to have no other transaction with them. They could not have a garbage deal at all.
When my dump starting filling up, the City to the North asked if I wanted to pay for that City dealing with my garbage overflow--- the water deal ended. However , If i kept the dump big enough to where they never made the offer, the water deal stayed on.
(If another City made the deal for the overflow of garbage, no problem.)

Every expansion on my map had to be first tied into my main water piping. No separate water towers. (Water towers before the first deal, no problem)

Once my City got above 125,000, the water deal seemed to be they wanted to sell me water. I finally had to break the connection. By that time, $ was not a problem.

Why go through this, It is pretty good money for an expensive City, early on.

Thanks for the tip, that's an annoying bug that was never fixed in the game.

So making a separate water connection with separate water pumps really fix it? Great to know.
It's an excellent source of income as you said.

edit: Oh, it has to operate on a separate energy power source as well? Interesting.