Does anyone know the "max wading depth" of MS and MX?
Same as any other car, I suppose. If it's more than 4 inches deep, avoid it. More than 6 inches, and you risk damage. More than 10-12 inches and you might get swept away.
Don't assume that since it's electric (and all motors are enclosed) that you can submerge safely. Be safe near flooded areas!
I haven't seen any data on Tesla when it comes to "fording depth", but you can assume that the depth is no higher than the "ground clearance" of any given vehicle, unless specifically stated. That's because vehicles that have stated fording depths (think Jeep) also have seals around specific sections of the drive train and other engine portions, as well as heavier seals along the bottom of the passenger compartment (doors). Without that, you're asking for trouble, even if you could conceptually make it through a higher water level.
Tesla drivetrain isn't fully waterproof, so at some depth it will short out and blow a fuse. What that depth is is unknown to me, but I expect it to be quite a bit over ground clearance, something like 10 inches. If there are exposed connectors lower than that they are protected otherwise so that it takes time before water gets to them.
I would like to see that tested though. Theoretically with full waterproof drivetrain Model S could go fully submerged.
@Timo: Give it a try and let us know how it goes. Might have to start calling you Nemo. :-)