I'm taking a cruise next month as well. I was planning on putting my Kindle in one of those waterproof bags and reading it on the beach, and not taking it our of the bag NO MATTER WHAT when I'm on the beach.
Just make sure you don't leave it out in the sun in any bag - it can quickly climb above (well above) a safe storage temp, let alone a safe operating temp. I'd ditch the black cover in the bag to keep down the temp and make sure it is in a bag or under a towel when not in use.
Any of the drybags sold for camping/canoe trips will probably work - it needs to be about 9x12 in size to allow it not to press down the keys from being too tight.
You could also use doubled ziplok bags.
For those worried about theft, you can often get extra policies to cover electronics (all the time or just on trips); your homeowners will probably cover it, but the deductible would not even be satisfied for a Kindle (it would for many laptops). Sometimes you can get a rider just for a trip that has no deductible and is reasonable priced.