I have found Progressive to be pretty on the ball with regard to this stuff.
One thing I have seen is when people fill out applications online, they don't always look closely at what they are applying for. They look for what costs the least.
Replacement cost policies are more expensive than actual value, but who but an insurance geek would know the difference between the policies?
The other biggie? "How much insurance should I buy"? You'd have to know the depreciated market price. Overestimating a boat's value just means you're paying more premium. Understating the value puts you in a co-insurance penalty situation (meaning you won't get as much as you think on the claim payment)....
So unless you have the marine "blue-book" with your boat listed, I'd contact your agent or some reliable industry expert to help...