This is from a yacht broker's site:

A Central Listing Agreement is entered into with just one brokerage firm, thus guaranteeing the listing agent and listing brokerage a portion of the selling commission regardless of what brokerage house brings the buyer. Co-operative brokerage agreements are standardized within the industry and allow one brokerage to sell a listing that another brokerage has Centrally listed. In this way, the Central Listing Agreement provides a maximum marketing incentive for the listing brokerage and individual agent, and encourages the promotion of the listing to other brokerages. Thus the marketing and representation needs of the seller are best met with the Central Listing Agreement.

The Open Listing Agreement allows the seller to list the boat with multiple brokerages at the same time and sell the boat on his own. The incentive being that if one of the brokerages with the Open Listing brings the buyer, that brokerage gets the full commission. The down side is, that the listing cannot be entered into the multiple listing data base, and most brokerages will spend the lions share of their advertising budgets on promoting their Central Listings, not Open Listings. Though on the surface, it often seems the Open Listing offers the opportunity for the most exposure, the opposite is most often true. The exception to this 'rule of thumb' is with the Mega Yachts. With an Open Listing on yachts that approach or exceed the million dollar market value, most brokerages will expend a substantial marketing effort regardless of the type of listing agreement. Even as attractive as that arrangement may seem there are still some down sides. The vessel would still require a Central Listing Agreement to be listed in the Multiple Listing Service. Also, with an Open Listing Agreement the seller will have numerous listing agents and brokerages to deal with, creating opportunities for confused communication and lack of a cohesive marketing effort. Buyers that see one boat marketed by several different brokerages usually think that there is something wrong with the boat.