Every boat has an intrinsic value that is not tied to changes in the market. I think $8K is a good value when you consider what they're getting.
No offense, but just because you think that, it doesn't make it true. If the boat has been on the market for a while, and you haven't found a buyer, then you are asking too much. The market sets the price, regardless of what you might think.
If you need a tax deduction, fine. Of course, the IRS is going to want you to be able to clearly explain how you determined the boats value. I can guarantee you that "I think $8K is a good value" is absolutely NOT going to satisfy them.
If you don't really need the tax deduction, then you would probably be money ahead to accept the fact that the boat is not worth as much as you think, and negotiate a deal with one of those "lowball" buyers.
Good luck, whatever you do.