I guess it is rather difficult to bring in to Belgium so much water by plane. Of course, if one has found the way to do so, customs dogs will have problems sniffing it! By the way, are you certain the fine was for smuggling water and not for transporting such a huge weight in her private car? Finally, Schengen has nothing to do with customs regulations, these are (for all practical purposes important for travellers) the same throughout the EU countries (Schengen-members or not such as the UK) but they are different in several non-EU Schengen countries such as Norway (see alcohol import restrictions). Schengen is about free movement across borders, i.e. passport and all other types of border control at mutual borders between member states have been abolished!!!