I guess your insurance history was good or else I would expect proof of value. Of course, I don't know what the agreed upon value is and don't want to know but it it were out of whack with BlueBook values, that would have raised a red flag.
Although used boat prices have risen as far as the insurance company is concerned, as long as they know their liability and are comfortable, they could care less if the boat was under insured. Believe it or not, the insurance companies often will use your credit score and automobile claim history as well as traffic violation history to determine boat insurance pricing.