In case of evacuation, flight crew can train and demonstrate their ability with a real plane and test passengers.
However; how can one be trained in case of ditching ?
Obviously, it would be quite problematic to ditch an aircraft just for training/demonstration purpose...
Flight crew training facilities have a big pool with a mock aircraft hull with slides and rafts.
Real aircraft are rarely used for training. It's much cheaper to build a fake aircraft interior into the building and use that than to tie up an aircraft that should be making money. It also lets the instructors simulate more dangerous scenarios, like smoke in the cabin, safely.
Actually, the FAA offers underwater egress training as a safety seminar. Around here it is offered in a high school pool, with a fake aircraft. You get will get totally wet, but it is good training. I first took a predecessor of this course about 35 years ago, when the Coast Guard offered it.
Look it up on the FAASTeam website, but here is an older link:
https://www.faasafety.gov/SPANS/event_details.aspx?eid=10024
One thing I keep in mind is that most of the water I fly over is 42F in the summer and colder in the winter. Not quite as nice as a pool. But going through the training, even if you don't like it, is a good experience to have when one is evaluating risks of overwater flights.
