Wow, hate to reopen an ancient thread here, but there are three questions involved here, I think, and they are equivalent to a car. A car has three types of insurance, normally. Comprehensive (and glass) covers your car if a tree falls on it. Collision covers it if you drive into the tree. Liability covers you if you drive the car into someone or something and cause damage or injury.
I am willing to self-insure on the tractor damage itself, but would like liability insurance for when I take it over the country road here, which I do occasionally. I don't think my homeowners liability ("personal liability") covers that at all. I would be willing to pay for collision and/or comprehensive coverage if I had to to get the liability, but my car insurance company doesn't cover tractors and I don't think homeowners' does either. A marine type policy is not really appropriate for liability because marine has strange liability laws. From what I understand, if you are at fault in a boat accident, if negligence cannot be established then you are only liable for the value of your craft prior to the accident. In a vehicular situation, or even say, the case of cutting someone's grass (as my sister's, not business) and, say the mower throws a rock and injures someone, I wouldn't be covered. This is the kind of thing that makes my blood boil. As an aside, homeowners' policies are skyrocketing in premiums, but if you look at your policy, you might be shocked to see that some things aren't covered. Floods never are, but things like earthquake or ground movement used to be and have been mostly excluded. Mold and rot have been severely curtailed in most policies that I've seen, often with only like a $10K limit. The insurance racket has become insanely crooked over the years, and it worries me, a lot.