The correct answer is B) show anchor lights, deck lights and cargo lights hung over the vessel's side. This is the correct answer because it provides the most comprehensive lighting setup to deter potential thieves while the vessel is at anchor. Showing anchor lights is the minimum requirement, but adding deck lights and cargo lights hung over the side creates better overall illumination of the vessel, which can help prevent unauthorized boarding or theft. The other options, such as only showing required anchor lights (A) or showing running lights instead of anchor lights (D), do not provide sufficient lighting to serve as an effective deterrent. Maintaining water on deck with firehoses (C) may be a good additional safety measure, but it does not directly address the issue of preventing thievery, which is the focus of this question.