A few weeks ago, I mailed a package to a friend. I decided to frank it using the new $2 Jenny.
I carefully removed the backing from the sheet and attached it to the Priority Mail box. I let my friend know in advance that I wanted her to send the stamps back to me.
Here's a scan of how it looked when it left my post office.

My friend carefully cut the box and returned the stamps. They escaped the marker monkey; BUT the quality of cardboard box wrinkled the stamps. I'm not sure if it was the handling, or if the porous cardboard absorbed moisture, or what?
This image is a photography from a 'side angle' -- a scan doesn't show the wrinkles.

It's probably a moot point anyway -- I don't know if they're soakable, and even if they are, I doubt the sheet would stay intact after soaking.
Kirk