Having worked in the mortgage industry, I have some idea of how underwriting a loan works. My guess is that the only reason they use NADA is simply carry-over from the car loan industry.
NADA is well known and well respected as a source of useful, accurate information when it comes to underwriting car loans. The people who are responsible for underwriting boat loans desperately want something similar, which takes most of the guesswork (and therefore the risk) out of approving a loan. They don't actually have anything that is as accurate as NADA is for cars, so they grasp at the next best thing.
When it gets right down to it, it is a CYA sort of thing. If the loan goes bad, the underwriter hasn't put his neck on the chopping block. He can always say to his boss, "But we only loaned him the amount that NADA said the boat was worth!"
Property is really only worth what someone can pay for it, so, what you are saying is that NADA is an accurate representation of what a boat is worth to the lien holder.
Most sellers or owners don't like to agree with NADA, but, in fact it is what it is. An accurate representation of what a boat is worth to someone who needs to get a loan.
Moving forward, since we may all have to sell our boat at some point, we need to stay in a price range that will allow the next buyer to finance if at all possible.
Most of us dump money into our boats that we will never get back but NO seller like to bring cash to the table to pay off a loan at closing.
So NADA may very well be in the ball park much of the time.