The correct answer is A) at least every hour on the hour in the open waters of the sea. The reason this is the correct answer is that maritime regulations require a vessel's dead reckoning position to be plotted at least every hour on the hour when navigating in open waters. This is a standard practice to maintain situational awareness and ensure the vessel's position is accurately tracked, especially in areas without visual references or automated position-fixing systems. The other options are incorrect because B) is too vague, C) is specific to coming on or going off soundings, and D) relies on GPS rather than dead reckoning, which is the focus of the question.