Renters and homeowners USUALLY cover a sailing vessel of less than 26 feet in length and powered vessels of less than 25 hp for liability exposure. So if you injure someone by running into them with your boat your personal liablility covers the injuries if they sue you.
Direct physical damage to the boat does not apply on the water. BUT if a storage shed or garage falls on it or burns up you have coverage USUALLY limited to $2000 for both the boat and trailer together.
Your auto only covers LIABILITY WHEN YOU ARE TOWING THE BOAT. So if you cut a corner too closely and clip somebody's car, your auto liability will cover the damage to their auto (first dollar, no lawsuit required). If your boat falls off the trailer or is damaged in transit, you have no coverage at all.
So - two things to consider:
2) physical damage
If your boat is older, just as with an oldcar, you may not want physical damage coverage BUT you probably should have a renters or homeowners policy so that you have liability coverage in case you hurt someone.
The only way to make sure you have physical damage of full value for a newer boat is to either buy a policy for the boat OR add a rider to your property policy IF your company does that sort of thing. You can also buy a boat related liability only policy for about $40 a year.
I prefer a real BOAT insurance policy in any case. BOAT US is a good source for these if your agent can't help you.
and, Nick, I still don't understand the extra charge on your auto policy unless they gave you some physical damage coverage to your trailer - which can be a good thing. Especially for only $10
I have been in this business for over 30 years - so ask anything you like