That is an interesting way to look at it, but really it assumes a broker is necessary for the boat - and that would be more up to the cat owner.
I _personally_ wouldn't be nearly so concerned about the costs to the other party in a sale, unless I thought I could get a significantly better deal by lowering their costs. Which is doubtfully, if they take the risk of going without a broker they will probably do it to save the cash for themselves.
It seems more like you are opposed to the costs involved - perhaps rather than a trade it would make more sense to buy / sell without brokers / realtors. Honestly it would probably be less work than negotiating a trade with certainly unequally valued items (especially after the boat value is modified due to anything in the survey). Certainly you are going to be picky about differing values if you are concerned about a personal 6% cost plus 2-3% extra you might get off the value of the boat for skipping the broker.
Though if you really want to trade - perhaps posting some flyers at liveaboard marinas might target the right audience.