Here is an update at where I'm at now:
It makes a big difference whether it is salt water or fresh water. If you have watched Car Guru's Youtube videos, everything in his car has an incredible amount of corrosion. Even his aluminum parts were highly oxidized. On my MS most of the aluminum looks brand new.
The worst damage that I have found so far was to charger and HV box under the back seat. It retained water -- for 9 weeks. Water got in but did not drain or evaporate. There was some corrosion in both boxes, but it cleaned up OK.
Next, the inverter and gearbox were both full of water. I drained them, and dried them but haven't disassembled them. I ran a scope in each one and the gearbox looks perfect, no rust. The inverter's boards don't have much corrosion, but the screws holding the boards in are very rusted. Each of these has a hole in the top, but no drain hole. On the inverter, I drilled a small drain hole on the bottom (per Ingineer) and another so I could feed in the endoscope. I have no idea if the inverter will work, as it sat in water for 9 weeks, but at least it is dry now.
The HV box on the passenger side of the frunk also leaked and retained about an inch of water. However the board looks great. No deposits. The one on the drivers side, DC to DC box I think, was dry and looks great.
The battery did take on quite a bit of water. My project over the last few days has been taking it out. It had about 1.5 gallons of water in it. I finally got the cover off today. So far looks great in comparison to Car Guru's salt water damaged pack. I'll post this in more detail later as I need some help from the forum on this. But I can back Richie up in saying, Tesla's batteries are not waterproof! I was really hoping it was.
On the 12 volt system, the worst boards were the Body Control Unit and the Driver's seat circuit board. Both sit low and the boxes retained a bit of water. These had the most corrosion, but cleaned up OK. Most of the 12 volt system seems to be working. I have both screens working, sound system, 3G, maps, three door handles present themselves, lights and turn signals all work.
As of yet, the lower power seat functions don't work (which makes getting the seats out really difficult). The upper seat and heat work fine. I thought the seat was on fire for a minute, but then my son pointed out that the heat was set to level 3. (I thought it was a driver profile for driver #3). GPS thinks the car is in Houston still, but maybe if I left the garage it would adjust.
Headlights and heater/AC I think involve HV, so I haven't got them to work yet. I also don't yet have a key FOB, which you probably know, is another can of worms. That and the belief that this was saltwater got me a big discount. Also the "unknown" odometer scared people, but it turned out to be 1,449 miles so that was another bonus.
Anyway, I'm still working through things, and not having the key FOB can be a good thing, as I am being methodical in checking out each system, before I try to take it to a supercharger.
The Interior cleaned up well, except for the seats and the tufts in the rear seat. I have peeled off most of the upholstery. I can't get the stains out of the leather in the seats. The cushions retain water and won't dry on there own, especially with the upholstery on. To get the water out, I have been putting towels on top and bottom and walking on them and then I keep a fan on them. . I
But overall, at this point, I have yet to find one fried circuit board or even a blown fuse. I am thinking that Tesla's system knew to shut itself down before any damage was done. The only thing that had to be replaced so far was the lens to the charge port that had melted. But I'll keep you posted, as I doubt that over two months of standing water on many of the components had to do some unseen damage.