Our rate is 6%, but it depends on what the seller has planned themselves. Sometimes we just help with valuations and a few advertising recommendations for the price of a beer or two.
A full brokerage package* (ie 10% worth) is getting rare these days, most people are able to sort out the photos and ads themselves.
Edit: A good specialist broker will not need to advertise the boat, they will call a couple of potential buyers and have an idea of the market appetite inside a week.
Edit 2: * By package I mean:- Finding a buyer, carrying out viewings and arranging for surveys, negotiating and then handling the funds through a client account. Possibly even sorting our delivery etc.