When the seller is not willing to negotiate a better price, the best move you can make is to walk away from the deal. When you walk away, you show the seller that you will not play by his rules and will not be toyed around during further negotiations.
After a few weeks or months, the seller will realize that no one else is looking at the boat, but the boat will continue to cost them money. If a seller decides to wait a year for a better offer, the seller would have to pay an extra year in slip fees. In a year, the boat will be older than it once was and worth less money, so any future offers will not be near the original asking price.
After enough time, a seller that was originally insulted by your low offer will come crawling back with a counter offer. At this point, the boat can accrue more problems, further lowering the value of the boat.
Most sellers get desperate during the Fall, when they realize that the buying season is coming to an end and they will have to pay to winterize their boat, as well as pay for winter storage. These owners want to unload their boat and tend to be very flexible with price negotiations.
It is safe to walk away from an old boat because not many people are looking to buy them. Old boats tend to have very few people look at them and the seller knows that the first person to put an offer on the boat tends to be the person who will buy the boat. If you are the first person, then there probably won't be a second person.
This means that if you walk away from a boat, the owner will sit around paying upkeep on a boat that no one else is looking at. After a while, they will get desperate and reach out to you with a counter offer.
Walking away from a boat doesn't mean that it will sell and you won't ever get to own her, it just means that you will own her later. It is important to remember that when you are searching to buy a boat, you are the rare gem, so don't let the seller make you fell less important during the negotiations.
State your price and negotiate from there. If the seller is not willing to negotiate, let them continue to pay for the boat until they realize that you are in control of the deal. If the seller doesn't understand that, walk away and show them that they do not have control over you.