Over here in Holland there are, apart from yachts, about 500 traditional sailing ships, some built in the 19th century, refitted from cargo barges to charter ships. A number of charter booking agencies promote the ships on german, dutch and belgian fairs and sailing events and handle bookings, apart from that there is a growing number of affiliate type agencies, and small companies aiming mainly for the business market.
The turn-over has not grown much since 2007 and most people that book a vacation are schools and groups that do so every year, often on the same ship or through the same charter office.
Yacht charters here are often handled by the yacht harbors, or by charter companies making deals with large general travel agencies as Fgual says, some run similar yacht charter offices in the mediterranean or caribean and use mainly social networking. There is an affiliate market in that particular branche, if you have the social skills and connections.