Let's say Amazon comes out with a new voyage with exact same specs as old voyage but with waterproofing. How many people would upgrade their voyage. Very very few. How many people who have been thinking about getting a voyage would be tipped over by the waterproofing? A few more, but still few. Most will just get the old voyage for a discount.
I think the number of people who truly want a waterproof kindle -- as opposed to the people who think sure, why not, or the people who want any excuse to upgrade (you know who you are
-- is very small. Even smaller than the number of people who want bigger than 6" screen.