One major difference between a marine insurance company and a car
insurance company willing to write a policy on a boat is that a boater
can buy a "yacht" policy through a marine insurance company.

Take the following scenario, for example:

Billybob Boater buys a Bubbly Bay boat for about $200,000. He insures
with a marine insurance company, and selects a yacht (or agreed value)
insurance policy. Two years later, ol' Billybob has a brain fart, runs
the wrong side of a channel marker, and rips the bottom out of the
Bubbly Bay boat and it bubbles down to the bottom of the bay. She's a
total.

The marine insurance company says, "Gee, that was dumb- but you insured
your boat for
$200,000 so here's a check for the same amount. May we suggest a
competitor when you decide to take up boating again?"

Now put ol' Billybob in the same Bubbly Bay Boat, with the same brain
fart and the same results, but let's say he saved a couple of bucks by
putting his boat into the same good
hands that insure his house and his car.

The insurance adjuster will say, "Gee that was dumb- but let's figure
out how to settle your claim....You bought the boat for $200,000, new,
and used it for two years.
According to our seagoing automobile depreciation schedule, a two year
old boat is worth no more than 60% of what you paid for it, so that
brings us down to $120,000. We can't just write you a check for the
$120,000, however, until we do an investigation to see what kind of
condition the boat was in just before it sank. You do take photos, with
a time a date stamp, every time you take the boat out, right? Do you
have the receipts for all the manufacturer recommended services?
They were on the boat? Oh, that's too bad............"

By the way, when the automobile insurance company wants to settle on
the mega-cheap, one of the tools they will haul out early on will be
the National Automobile Dealers Association's opinion of the value of
your boat. (See NADA book remarks in the "used boat price" thread). :-)