A diver with the correct cleaning gear can do the job considerably more quickly and cheaply, and with far less hassle.
Just because boats are in the water doesn't mean they don't get dirty. The underside of a boat can get encrusted with marine life and aquatic scum over the years, which needs to be cleaned off.
Consult a reputable marine establishment for advice on the appropriate boat cleaning equipment to use, then begin marketing your business. Have business cards printed and distribute them to boat owners, as well as posting them on notice boards at boating shops, chandlers, and yacht clubs.
You could also advertise in specialist magazines, ensuring that the boat cleaning rates you charge are noticeably cheaper than traditional dry dock methods.
Keep good records of your clients. Once you've built up a client base you could contact them annually to suggest they have their boat cleaned again.