The OP (assuming that he is still monitoring this thread) can count me as one more person who believes that this was a case of vandalism/sabotage that took place after the car was parked.
There is <b>no way</b> that the engine would have continued to run for 6 miles after the tank was filled with "fuel" that was 75% water.If the OP is in Flint,MI the lead in the water would ruin his catalytic converter.
I remember one lady who encountered water contaminated gasoline on her 1 year old Subaru. She filled up and the car died as she pulled off the lot. It was towed in and upon draining the tank I found about 3+ quarts of water. Once the lines were blown out and the fuel filter changed along with some fresh gas it sputtered for a moment and then ran fine.
She was given a statement to use against the gas station as that amount of water could very well be due to underground tank contamination. She was told to find another gas station for future use.
She went on vacation the next day and her first act upon returning was to go to the same gas station as before. This led to the car dying immediately. Repeat the above.
Point here being that 3+ quarts of water in a full tank did not allow that car to travel even half a block.
Water always settles in the bottom, the pump pulls from the bottom, so any problem is going to surface almost immediately.
To continue the great mystery, the OP claims they put 12 gallons of gas in the night before. Assuming an 18 or fewer gallon tank, how would a vandal get enough water in the tank to yield 75% water? You have to figure that 12 gallons pretty much filled the tank. So then we are left with a thief who stole the gas in the tank and replaced it with water? I find that a little hard to believe that “he” would carry around a couple cans of water plus a couple empty cans to drain the gas out into before politely putting the water in to cover his tracks? I’ve never come across such a thief.
Now maybe the 13 year old son, if there is one, went for a joy ride while the OP was asleep and then re-filled the tank with water to cover his tracks, not knowing that cars don’t run on water. Now that I could believe and the plot thickens. That’s my theory and I’m sticking to it.
I have to suspect the 75% claim by the mechanic as being a bit iffy seeing as how he apparently never drained the tank.
Theorizing a bit, it might be pointed out there may have been a small amount of water in the gas when the tank was filled and the water molecules had not gone through cohesion due to the turbulence in the gas pump and bouncing around on the way home.
In theory, the car could make it home with little or no symptoms until it sat overnight and the water molecules started searching each other out to form an unruly mob… 
Correct me if I am wrong, but doesn’t gasoline float on top of water? If so, not only does that call into question the percentage of water vs gasoline, but also the 6 miles driving - that is, the engine pulls fuel from the bottom of the tank, so only the gasoline in the fuel line line would be available.
Nevertheless, the OP needs to purge the entire system and hope for the best.
You don’t need a lot of water in the tank to freeze the line.
Yes @Triedaq, but the lead would be great for classic cars that haven’t been converted to unleaded gasoline.
I don’t expect a return by the OP. This is no doubt a completely fabricated “drive by” tall tale created by someone with zero knowledge of the subject. US military aviation is absolutely anal concerning fuel contamination. I was never a fueling specialist (commonly known as gas passers) but they were members of my flight operations section. I have always believed if you are supervising someone you need to have at least rudimentary knowledge of what they do. When I spent a day with them I learned that first thing in the morning they tested the two 10,000 gallon above ground fuel storage tanks. One test was the “aquaglow”. They drained a sample from the tank bottom and used a device that would detect any water contamination. This test was repeated on the two 2,500 gallon tanker trucks. The crew-chief’s aircraft daily inspection required a sample in a clear glass jar that had to sit for 30 minutes and be visually inspected for contaminants. The FAA (civilian) fuel storage tank inspections are only required every 30 days. I revived a 1956 Ford pickup that had been vandalized with a gallon of anti-freeze in the tank. It had ran for less than 3 blocks! I drained and flushed the tank. Flushed the fuel line using the starter motor. Cleaned the float bowl and spark plugs. It ran rough and smoked for about 30 seconds but completely recovered.
Has anyone else driven a vehicle with a sediment bowl? I recall a Briggs & Stratton push mower with a sediment bowl fuel filter and oil bath air filter. And small planes once had petcocks on the fuel tanks that were opened to feel for water during the pre-flight.
And water in fuel in my area is most often the result of melting snow being dammed up over the fill tubes which are often not properly sealed.