Already waterproof.....
https://www.youtube.com/watch?v=4vGcH0Bk3hg
In answer to some of the other questions, I understand they don't intend to ever land them.
I believe the package will be lowered?
https://www.youtube.com/watch?v=w2itwFJCgFQ
If you watch the TED talk video I posted above you will see a quadcopter having two of its props cut off with scissors and still flyable in a emergency mode with all axis control possible.
Whether it all happens, who knows, but I do know that for it to be worth doing they have to be autonomous to make it worth the effort. If each one has a pilot then there is no saving.