It's a high-end house; the owners were clearly art collectors, the bathroom probably had state of the art ventilation and climate control and I am so overthinking this.

There would be a small degree of protection afforded by the frame, but that would only be for the edges and corners of the painting.
More security could be added by a glass over the face of the painting, especially for a watercolour, but obviously that was not part of the gag
^-^
The Opener of the Way is waiting.

I thought so too but even if it was in a frame, they are not water tight are they!?
wat are you lookin' at

Hahaha! Good point, what WERE they thinking. I just tell myself, 'it's only a movie, it's only a movie!'.
Martha
Austin, Texas