The correct answer is C) Water depth of less than twice a vessel's draft. In the context of shiphandling, shallow water is generally defined as water depth that is less than twice the vessel's draft. This is because when a vessel operates in water that is less than twice its draft, the water depth can significantly affect the vessel's handling characteristics, such as increased squat, reduced under-keel clearance, and changes in vessel stability and maneuvering. This can make the vessel more susceptible to grounding or other navigational hazards, requiring the operator to exercise additional caution and skill. The other options are incorrect because they do not accurately reflect the standard definition of shallow water used in the maritime industry and on Coast Guard license exams.