At least around here (CT) boat brokers are not like house realtors. The boat broker only gets a commission for sure for a boat that he lists. So when you tell a broker your requirements he will show you a very small number of boats that he represents not everything in the area.
There are exceptions of course and if you happen to develop a relationship with someone they can do a lot for you even help you buy a fsbo. There are basically no such thing as a buyers broker and sharing commissions is done but not common.
You are buying a boat not a broker you will most likely get to know a lot of brokers and see a lot of boats.