One advantage to a buyer broker is that you will gain access to the Yachtworld sold prices. I don't know how you can obtain these otherwise. In some cases, these may provide a good deal of negotiating ammo.
Personally, I see nothing wrong with an offer of 45K on a boat listed at 64K. In fact, that may well be a good price for the seller in this market. Whether it is a "good" offer or a lowball will depend very much on the make and condition of the boat -- some boats have more active markets and predictable price points.