Is $890.00 a fair price to get rid of water in the gas tank?

What is your question?

So after getting not even a full tank of gas at a hole-in-the-wall station, my car failed to start. After a few minutes it did and after getting a mechanic look at the spark plugs everything seemed fine. After a day the idle sounded rougher, two days pass and the Check Engine light comes on. Then it nearly dies while idling at a grocery store. I'm now being told by a mechanic that I have water in my tank and that he wants to bill me at $890 for dropping, cleaning and replacing the gas tank, replacing the spark plugs and cleaning the throttle body. They haven't started the maintenance yet and no money has exchanged hands, I just want another option on the price versus the work outlined.

How long has your 2014 Toyota Corolla had this problem?

3 days