When going down inside the boat, what direction are you moving?

Moving down inside the boat is best described as going "below deck." This term specifically refers to the action of descending into the enclosed space of the vessel, which is typically located under the main deck. This space may contain sleeping quarters, storage, and other facilities necessary for the operation and comfort of the crew and passengers.

The use of "below deck" also indicates a vertical movement rather than a horizontal one or movement toward a specific area of the boat. The other options pertain to directional movement within the boat. For example, "aft" refers to the back of the boat, "starboard" refers to the right side when facing the bow, and "bow" indicates the front of the boat. However, none of these terms accurately describe the act of going down into the internal spaces of the boat, making "below deck" the appropriate choice in this context.