I agree with the assessment of the car and will add this. Don't pay a lick of attention to any appraisal, especially the seller's, they are almost always way higher than market value. Look around at what similar cars sell for and figure no more than wholesale because you now have to still sell something to get your money.
I recently had to nurse a friend through a similar situation as he was trying to unload an aging airplane in a plummeting market. He considered many offers to take partial trade so he could make the sale. One in particular was a 54 Belair 2 dr hardtop. I helped him evaluate the car and they figured it was worth about double what it would actually sell for.
That being said, it is all relative. If you are getting a good price from the boat and the trade value of the car is very reasonable then that is attractive. And if you actually like the car and want it then all the better.