FWIW, when I called and spoke with a BoatUS insurance rep in 2018, the nice lady upped the Agreed Value of the policy on my '88 boat by the price of my new $10K engine/drive train replacement. That was parts only with no allowance for labor.
I have been with BoatUS for over a decade. I offered to send a copy of the engine invoice and was told they would take my word for it. Since insurance is priced out on a premium cost per thousand basis, this did not change my premium too much either.
Our boat is insured for (almost....) what it would cost to replace this exact kept-up-to-date boat, and not what an unmaintained one would sell for, I should point out.
I am not sure I follow the thread discussion about % payment for a repair. I might be missing something.... and now you all have me feeling a tad concerned. Hmmm...