I can see how water+dust proof could help if you read on the beach, but I would expect wind-blown sand to sand-blast & ruin the screen even so.
an outdoors reader needs to survive living in a hip pocket, with the possible torsion/twisting effects that can have on the device as you move about. I guess that current e-ink screens are too fragile for that.
PS there's a difference between water resistant ( can survive a little rain) and waterproof ( being dropped in bath/swimming pool ). waterproof requires a cunning solution for any USB or SD ports.
[ i do have a washable PC keyboard which copes with that last point by having a cap that goes over the usb cable end). I have not actually submerged it yet, but allegedly I can.- but the instructions say I have to leave it for several hours afterwards to drain & dry out, before re-connecting it]
http://www.logitech.com/en-gb/produc...-keyboard-k310