A Silly Spook May Sit Bayou Side
The first thing that needs to be said is that the concept is not ridiculous. The high water table in New Orleans makes the idea of an old plantation house partially submerged in a bayou a plausible enough setting. In fact, the ride's graveyard scene was originally going to be a swampier affair, with bogs and frogs and trickling streams. (A trace of this remains in the tea party tableau, where a hearse is supposedly stuck in the mud.)
The problem is that practically nothing has been published in the way of artwork, whereby we may satisfy our curiosity. But hey, since when has lack of evidence slowed us down? At Long-Forgotten we laugh at lack of evidence. Ha ha, laugh we. Anyway, it turns out there are clues to be found in various crooked fannies, and these clues will allow us to at least raise our guesswork to the level of informed speculation, even if a goal as modest as simple probability remains out of reach.
If Coats and Davis were involved, and Walt vetted it, we can probably assign a date of 1964 or 1965 for Fred's brainstorm. Judging by the number of Imagineers who worked on it, the idea must have remained afloat for a respectable length of time before it was finally abandoned. As we shall soon see, this could be useful information.
Only in the Movies
A partially flooded Mansion was realized at least once—on film. The 2003 Haunted Mansion movie makes use of a concept similar to Joerger's. After a heavy rain, a huge underground crypt is flooded and serves as the locale for a major scene in the film. Presumably, the basement levels of the house itself would also be flooded, although you never see this in the movie.