That is a good technical question. Typically, the broker can not commit on behalf of the owner, so they are not technically an agent. In standard contracts, they are often listed as an independent third party with their own rights and responsibilities.
Here is the definition of "Agent" from Black's Law Dictionary: "A person authorized by another (principal) to act for or in place of him; one intrusted with another's business." (Black's Law Dictionary, 6th Ed. 1991).
A yacht broker is most certainly either an agent for the seller, an agent for the buyer, a dual agent, or, by contract, an independent contractor rendering limited, specific services in connection with a transaction and not an agent for either party, if allowed by statute. Although there are uniform laws on agency and partnership, state law will govern the licensing and duties of agents transacting business within the particular state.
As a buyer looking at boat listings, you should reasonably assume the listing broker is an agent of the seller until you discover otherwise.