Since you are doing a "long distance" purchase (none available in your area), a buyer's broker may not be much help. They would split the commission so they are less likely to hear back from the listing broker than you. Long distance buy is tough. I have tried it and learned that, before you travel, you want a local surveyor to take a prelim look. I have done this a few times and for $100 had an independent (not recommended by broker) surveyor do a first pass.
I am really surprised that listing brokers are not responding. Certainly many are on vacation but a well run company would communicate that information. I do not typically ask for the listing broker, but ask for info on the boat. Many times I learn the boat is sold or off the market. I might also learn the broker quit or is out of town for 3 weeks. Many brokers will not waste their time calling a distant buyer ( low percentage of selling) to try and establish a connection. How many times have you gone to a broker to ask about a boat only to have him show you something completely different sitting at his dock ? Many are like car salesman - immediate gratification, no long term view. NE is unusual because we have a better bunch. IMO - they are knowledgeable salts with solid ethics. Try California or Florida if you really want some BS.
If you want to explore the buyer's broker option, you could consider one of the quality NE brokers in Maine or Mass. They can get it done, but they will need to believe you are a serious buyer with cash in hand. I can give you a few names if you like.
"We have met the enemy and he is us." - Walt Kelly's Pogo