From my experience, book value on boats has to be taken with a grain of salt. Boats hold their value so much better than cars. To me, it seems NADA and others depreciate them like cars. I paid more than book value for my current boat, and for two of the boats I bought in the past. One of them was a 20 year old 50' Gibson houseboat. When I sold it it was 27 years old. I sold for 18% less than I bought it but still sold for more than 'book' value.
The other one I paid over book for was a Force 235 Baja. I had it 17 years and put over 1500 hours on it. Fantastic boat. The 454 motor blew up and like a dummy I traded it even for a corvette. A poor trade on my part.
I hope this helps a little bit. Paying more than book is common. Selling for close to what you pay for it will depend on how long your keep the boat.