The correct answer is B) speed of the vessel over the ground or through the water. Vessels required to have an Electronic Plotting Aid (EPA) must have a device that can indicate the speed of the vessel over the ground or through the water. This is a key requirement for an EPA, as it allows the vessel's navigation system to accurately track the vessel's movement and position over time, which is essential for functions like collision avoidance. The other options are incorrect because they do not reflect the primary function of an EPA. Option A refers to relative heading, which is not a required capability. Option C refers to vessel position, which is important but not the core function of an EPA. Option D refers to water depth, which is not directly related to the purpose of an EPA.