The correct answer is C) tap the bulkhead with a hammer to check for a water level. To determine if a compartment has flooded, tapping the bulkhead with a hammer is the best method. This allows you to listen for a change in the sound, which would indicate the presence of water behind the bulkhead. Opening the hatch (option A) or the watertight door (option D) could allow more water to enter the vessel, which should be avoided. Feeling the bulkhead for heat (option B) would not provide a reliable indication of flooding. The reasoning behind this is based on standard marine damage control procedures, which emphasize the importance of carefully assessing the situation before taking actions that could further compromise the vessel's integrity.