The bag is relatively spacious. Extends roughly equivalent to the size of a protective bag an SLR with standard lens. Otherwise half. Caution: If you have a battery-operated rear light mounted on the seat post, could possibly get problems with the place here.
Issue of water from above and below: rain and spray water are not a problem. Go scuba diving, but you can not with the bag. Here gives, as mentioned by another reviewer, a freezer bag as a second skin inside additional protection. Admittedly, this is only necessary if one transported in extremely water-sensitive objects (eg mobile phone).
