Although commissions are "negotiable", you will find that they vary between 6 and 10% for the most part.
Some brokers actually work for you, others not so much...one I worked with last would often hand me the address and say good luck. When I would call for directions or details...he never knew anything or how to get there, or "I have not seen the boat" -even one of his listings. If it was not at his pier, he would not show the boat.
Others will meet clients and take them to your boat, and sell the boat as much as possible....at least they will know the details of your boat, or should.
As a buyer it is often good to not have a good/interested selling broker, as a few years back I looked at a boat with the selling agent absent, made my offer, it was accepted. When the selling agent went on the sea trial with me and the surveyor, he must have said a dozen times, " I didn't know this boat had a" generator, air conditioners, NEW sails, auto pilot, in such good shape, etc, etc
YMMV, but in this market I would make sure that my broker was the energetic, active one - interested in selling my boat.
all the best