Door rust... (Where is this coming from.)

[Er_Ras]

I have an 81 300d. I'm currently working on getting the engine going, but in looking at it I have a lot of rust at the base of doors on the inside, below the door panel, and where the door meets frame along that seam. (Pulling up floor mat/rugs and searching around elsewhere I really don't see any rust. Looks like the frame has an undercoat also.)

The car came from an area where there was lots of water on the roads so I'm just curious about what failed that let it rust so bad there...

Things that I can see that could be a potential issue, yet none of them look bad enough to really cause this:
1: Windows seam's leaking and water sitting in the door.
2: Seam between door/Floorboard hard.

To me where the rust is seems a bit weird, as the very bottom edge of the door doesn't have any rust on it though.

(Is this a common problem that needs to be watched? )
Jp