Engine knocking after stalling in high water

1999 Cadillac Seville STS, 138,000, V8 32v NorthStar
Hello,
The other day I drove through some high water, it was higher than I thought but I was behind a pick-up so it didnt look as high because the truck was throwing all the water aside. The truck turned off midways in the street and the water just rose up on my car and I felt the engine about to cut off, so I tried turing up into a higher surfaced parking lot, but the car died while I was on my way up the ramp. Knowing that nobody was around and I couldnt push the car uphill on my own especially in water, I attempted to start the vehicle 2 or 3 times, and planned to put it in drive quickly to boost the car up to the dry surface. After a few tries, I finally was able to get the car to stay on and drive up into the dry land. As soon as the car turned on, I could hear a knock in the engine, and a big loss of power, then it would have a surge of power then go back to wanting to turn off, so I left the car there for a few hours until the water went down. The car is now in my drive way, but when I turn it on, the knocking is very loud, it sounds like when you have no oil in your car. When I went to check the oil, it seemed as if there was no oil, so I put oil and it took only 3 quarts, so it wasnt real real low, but still lower than normal. The car hold 7-8 quarts. When I added the 3 quarts, the knocking sound quieted down a bit, but it just came back after a while. I put some water treatment in the gas and drove the car to the gas station to put in a few bucks for the treatment and the car died out a few times. The check engine light is on, but im afraid to drive it anywheres. Yesterday I moved the car back to get into my garage, I only hasd it on for a few seconds, but when I got out of the car, I saw a fresh oil spill on my drive way.
Just to let you know, when I first ran through the water, the car was still cold as it was sitting for over 2 hours, and it was only running for maybe a minute when I hit the water.

What could I have damaged? What can I do to check?