Why is the floor of a Mercedes 1985 300 TD under 2 inches of water?

It may be your outside window scrapers. Look for a gap between the door Windows and the rubber scraper. If it is missing, torn, or worn out, it will let water into the door. That, in turn, will allow water to get on the floor. Inspect the bottom panel of each door. If it appears to be leaking water, open the door and shake it back and forth, listening for water (if it's been a while since it rained, wash your car). Performanceproducts.com offered replacement parts for W123 cars.