PS Home insurance says in writing they cover half mil if I sink someone , run over a skier etc but zero if I wreck their boat.
Sounds right, and that's the big worry. If you trash a lower unit, figure you're on the hook for $2k. Most damage a cautious newby will do is about $2500, either that or it's a total loss by a moron. I ran Balboa Boat Rental in Newport Beach many moons ago (I just saw a picture of me from back then
), and a boat rental on Table Rock Lake, and unless you did something supremely stupid, they never incurred extra costs. As many bottom patches, lower unit/out drive, rigging repairs I've done, we probably only billed two. One guy who sailed under a bridge he was instructed in no uncertain terms, I even pointed at it when I was checking him out, "Do Not for any reason go under that bridge. That and the Kimberling City Bridge are the boudaries of where you take the boat. Well, he sailed to weather towards the bridge and figured "WTF, I'll fit fine" sails up a bit further turns downwind and breaks the mast in half on his way through. We billed his Visa card for that. Another guy was drunk coming back into the marina and ran over the breakwater at speed tearing out an IO drive at the gimble. We only billed him for a drive, not the repair or the bottom repair.
Most rental agencies will brief you adequately in the operation of the vessel and be able to provide the required local knowledge. Good cartography is nice to have as well.
PS, if it was a rental golf cart, it would be covered. In TX adjusters class we used to make jokes about the insurance commissioners all being golfers, because your homeowners liability covers pretty much every sin you can commit on a golf course.