Easy. Go to a marina and ask for the value. They have a book and can look it up for you. Now is not the time to be selling unless there are other factors at play as most boaters are putting theirs away for the winter.
The "book" is really worthless. It covers only "average" values. A salt water boat is worth a lot less than a fresh water gem from Minnesota. Regional values are the only criteria.
Also, marina quotes are second to worthless. Maybe if you tell the guy he can take it on consignment and have a big commission he might be willing to work with you. But, many of them frown on giving free advice.
I sold my boat late Summer this year. I knew what the NADA value was because I looked it up myself (fancy that). That info was backed up with weeks of examining local CL data.
I also went to a local marina and offered to have him sell it for me. He wasn't interested because he already had too many used ones on the lot. He quoted me a price and said that's all it was worth. It was HALF the NADA value.
I ended up selling it in 4 days, to the first person that came to look at it, for 3 times NADA and 6 times the dealer price. Of course, my boat was pristeen. I think if it had rot in it and I was concerned about safety I would have donated it to a local MV charity and taken the tax deduction, just for piece of mind.
If you want anything done right, you need to do a lot of R&D.