What is the naval term for 'below'?

The naval term commonly used for 'below' is indeed 'downstairs'. In naval terminology, 'downstairs' often refers to lower decks or compartments within a ship, differentiating levels on board. It signifies the areas that are lower than the main deck, aligning with how ships are organized into various layers.

The term 'deck' refers to the structure on which crew and passengers move but does not convey the idea of being below or underneath it. 'Above' clearly indicates a direction opposite to below. 'Underneath' might be colloquially understood to mean below, but it isn't a term typically used in naval jargon or formal communication within the maritime context. Thus, 'downstairs' is the correct terminology aligning with naval customs.