I've got two waterproof Kobos, but it's not a important feature for me (and the seal on my H2O doesn't fit firmly anymore anyway).
In actuality, my waterside device is my old Mini for the following reasons:
- portability
- it gives it a use
- it can live in my beach/pool/lake bag (not otherwise in use + great battery life)
- it wouldn't break my heart if something happened to it
It's loaded with short story collections and lighter fiction which make for better waterside reading so syncing's not a issue.