There is a cost to everything, especially relating to boats. Nothing is "free", including the services of a 'buyer's broker'.
My understanding is the same as others have said, that a buyer's broker would split the commission with the seller's/listing broker, thus generally 5% each. The pitch is that it 'doesn't cost the buyer anything', which may or may not be true.
There have been a few boats I've made offers on over the years where we couldn't agree on a price (big surprise), but weren't that far apart. In more than one case, the seller's broker offered to reduce their commission to 5% to try and make the deal happen. Their thinking was that since I was not working with a buyer's broker there was no one they had to split the commission with, and they were willing to take 5% rather than nothing to get the deal done, a very enlightened and respectable attitude. If I had been working with a buyer's broker it's unlikely they would have made that kind of offer.
However, there have also been other times when I've suggested to a seller's broker that they reduce their commission to try and make the deal happen and they weren't interested at all, and were looking forward to the 'windfall' of getting the entire 10% commission for themselves. I also imagine there might not be too many cases of this happening in the current white-hot seller's market where many buyers can't throw money at the seller fast enough to get the boat.
I'm open to the concept of a buyer's broker, in the interests of saving me time (which is the most precious and limited resource, especially at this point in my life). Pretty much every broker I've ever talked with has pushed hard to 'represent' me as a 'buyer's broker' with the same pitch 'it won't cost you anything'. I can understand the appeal from their perspective, to try and lock in a 5% commission on a deal. I've tried working with a few, but my experiences have been that all they seem to do is send me lists of yachtworld listings of boats I'm already aware of, to try and claim a commission if I end up buying any of them. I know how to use a computer and am very aware of what's listed on yachtworld and boattrader, and it's of no value to me to re-send to me listings I've already seen. When I've asked for help beyond just sending yachtworld listing, so far the responses have generally been the sound of chirping crickets. I'd love to hear of positive experiences and true value-added buyer's brokers, but I haven't experienced them myself yet.
Ultimately, the buyer pays, for everything. The money in a deal has to come from somewhere, and it comes from the buyer. There are all sorts of ways to cosmetically slice and dice it to make things look different (splitting commissions, price reductions, 'credits', etc.), but at the end of the day, the money is coming from the buyer's wallet to be divided up among those involved.