I've done both, sold with and without a broker.
Whether using a broker will work for you depends on a lot of factors, the big two are time and effort. It will take time to effectively market your boat, placing ads (and paying for them), answering emails, and the most time consuming is showing the boat. If you live close to the boat and if your schedule is flexible, then showing it may not be much of an issue. On the other hand if you work or live some distance from the boat this can be a highly time intensive endeavor, not to mention frustrating when you get the tire kickers who have little intention to buy.
A broker will give you a wider market reach even if they do not actively market your particular boat. The boat will be listed on the broker's website and on YachtWorld.com. The broker will also handle all the paper work, so you will not have attorney fees nor will you have additional marketing fees. The broker can also coach you on how to make your boat more salable. And the broker wastes his time on the tire kickers, not yours.
Perhaps the greatest value the broker has is dealing with issues if something threatens the deal. He is a buffer between you and the buyer and can help negotiate a resolution.