Last year the surveyor valued our boat at $10K Less than what we agreed to pay. He used sold boats. I had my buyers broker also give me data from sold boats. I derived a higher figure, after adding in equipment others didn't have and downgrading for a paint job I knew the boat would require as well as for new batteries and new exhaust elbows. The surveyor left out what for me were significant issues.
For example, the PO left about $$5-$6K in spares and two brand new props. The surveyor added no value for these saying, who knows if you will ever use any of it. I have of course used it plenty, and will, I am sure be going back to the 15-or so tupperware totes for other things. The surveyor also didn't add as much value as I would have for such things as stabilizers (hydraulic and flopper stoppers), extra generator, extra ice maker in flybridge, two ACs in fly, etc. The boat came with a ton of other extra equipment that I thought had value and that I would have had to ad, if it were not there. The broker did not seem to think that it added much to the value.
Lessons learned by me. I was told by the buyer during the handover, that he was going to keep the boat had I tried to stand by the surveyor's price. (Who knows how he would have felt two or three months later.) I suspect (now) that I might have gotten the boat for $10K less had I held fast and decided to wait. (Again, who knows. The PO's broker told my broker that the boat would be listed for $30K less than the listing price if I walked, which I thought was a threat with no teeth.)
In the end, I remembered what a friend and real estate broker told me when we were buying a house in 2000 for a bit more than a quarter mil. I was fussing about $10K and he said $10K in a couple of years will seem like chump change. Who is to say, he said, if the house is really worth what we agreed to pay or $10K more or less. After having lived a few years more I tend to agree.
Now to your other questions. Our insurance company would let us insure the boat for $15K more than the surveyor stated. We were told to keep track of receipts and or get the boat resurveyed after a paint job. BucBoats listed our boat's value, without all the extra gear, at almost twice the price we paid, as did BoatUS, which will also give you an estimated value. Our POs had the boat surveyed for insurance five months before we bought and the value came out at more than $50K more than we paid. So, as you can see, a big range of values from professionals.
Boats are so individual, from equipment to condition and previous care, that assigning values, in my mind, is pretty subjective.
If you have a buyer's broker, he or she ought to be able to provide sold boat data. Sometimes notes about the boat's condition or special conditions of sale are noted. If you get that data, you will also see high prices for boats that were part of a trade in and you will see some lower prices where POs were in a hurry to sell.
Good luck,
Gordon