We charge 10%, but will work with you. I always ask what you need to put in your pocket and work around that figure, as long as it is in line.
A dealer will advertise your boat which costs money of course. He also has a major edge, as he can arrange financing and handles registration work and documentation issues.
You definately get more exposure this way. Plus you don't have to answer the phone and wait for people to show up. A lot of times they don't!