Been there, done that. The "umbrella carrier" always requires you to
carry some minimum amount of liability on your house, cars, boats,
etc. before they will issue the policy. Typically the minimums are in
the range of 100 to 300K$.

I have an umbrella liability policy that covers risks for which there is *no*
underlying liability insurance available. I have had the same policy for over
20 years, (although the limits and premiums keep increasing), so things may be
different in the current market.

Suppose, for example, that I owned income producing real estate. At least in
my state, many insurance companies will not write fire insurance on older
rental dwellings. One that will write fire policies on 20-50 YO rentals does
not offer inidividal liability insurance for houses or small multiplexes, but
does offer an umbrella policy that covers not only property on which they write
fire insurance but any other asset or activity as well.

If I owned income producing real property in my state, had a policy like I just
described, and had a 26-foot sailboat that was otherwise not insured my
liability would be covered while sailing.

Every situation is different, and the guy contemplating buying the "super
cheap" sailboat should be sure he explores all the options.