Why the shoes off unless they were expecting to waterland?
Nothing to do with a "waterland". It's essentially required to ensure that the heel of a shoe (especially a lady's high heel variety) doesn't tear up the slide should it need to be used.
However, I'm not entirely convinced that it's a good idea to escape from a wrecked (if applicable) aeroplane shoeless!
Edit: 757 driver's fingers were quicker than mine.