We went thru the process of booking a crewed charter twice. Once via captains and the other with a broker.

Absolutely use a broker.

The difference I noted was a broker worked to find a boat and crew that was a fit for our group/vacation style while the captains seemed to want us to fit their boat/style.

(Full disclosure - we didn't take either trip as when it came right down to it, my group is land-based at heart and wouldn't have liked a boat trip.)