What to do if I bought gas that had water in it and as a result I suffered great harm to my car?

You could potentially sue, and recover the cost to repair your car, from whomever was responsible for the gas being watered that could be the gas company, if they manufactured watered gas, but this is unlikely it could be the distributor, if they watered the gas before delivering it it could be the station owner, if she/he watered the gas etc. A good way to start may be to sue the station and/or station owner in small claims court acting as your own attorney to save on legal fess this called representing yourself "pro se", since they are presumbly local, and you can generally only sue local parties in small claims--to sue a national company, like Exxon, say, or a non-local distributor, you'd probably have to sue in regular county or district court, which can be much more complicated and costly. Also, it most likely the case that it was at the station that the gas was watered. If you can show that the damage was caused by watered gas and that you purchased it there, you could recover compensation. You could use your own testimony as to where you bought the gas and when you discovered the problem, but would likely need to put your mechanic on the stand to testify about how he knows in his expert opinion or experience that the damage was caused by watered gas and what it costs to repair.