My insurance policy covers me for 3rd party injury and liability and legal expenses, but I think you have to check individual policies/companies as not all are guaranteed to.
My insurers asked engine size/power, boat size and max speed of the boat. This won't guarantee that they know the maximum power engine for a size of boat though, and I'd guess they'd look to wriggle out of paying if your boat stated 100hp max and you had a 115hp fitted (for example). Also to add further confusion, the same boat may be fitted with a larger engine if a different year, for example my 2003 Ribeye A600 is rated for a 115hp engine, but later models (2006 onwards I think) are rated for 150hp engines.
My boat/trailer is covered under my boat policy for storage, transit and launch/recovery providing the trailer and tow vehicle are "fit for purpose", in a good state of repair and all necessary precautions are taken/applicable regulations followed. Trailer to have hitch and wheel lock fitted when unattended. My car insurance covers me to tow a trailer that is fit for purpose, in a good state of repair and conforms with legal requirements, etc. But I'm not sure the car insurance would cover the load in the trailer (not an issue since this is covered by the boat insurance).
There's weather out there - must be time to RIB!
(Or dive, or ref rugby, or.......)