Final answer:
The registration number must be displayed on both sides of the bow of a vessel. It is a legal requirement that aids in vessel identification and must be in a specific format. Other items listed are not required to be displayed on the bow.
Step-by-step explanation:
The item that must be displayed on both sides of the bow of a vessel is the registration number. According to boating regulations, this is a legal requirement to help identify the vessel. The registration number is unique to each boat and must be displayed in a specific format, which typically includes block letters that are at least three inches high and contrasting with the color of the boat. The other options, such as boater safety certification card, vessel capacity plate, and owner's name, are not required to be displayed on the bow of the vessel. The boater safety certification card is usually carried on the person, the vessel capacity plate is typically attached to the vessel in a location visible to the operator, and the owner's name, while it may be displayed for personal or commercial identification, is not a legal requirement on the bow.