Many many cruisers don't have comprehensive insurance. They tend to have oversized anchors, sail with a defensive mentality, are less likely to abandon ship and have a reserve fund for the days something goes wrong. This is pretty essential.
There is a recent saga where a couple grounded their boat, had no such fund and could not pay the locals the towing fee demanded. They finished up selling the beached boat for $3.
I am not sure about the US but in the UK basic 3rd party liability insurance is available without a survey, this gives the evidence of insurance that's required for boat licence renewals or where evidence of third party insurance is required by a yard, marina or boat sales company.
Navigators & General is now able to offer third party only insurance from just £58.30 a year. This is an online only insurance policy for UK boat owners.
No survey requirements or costs
£3,000,000 liability protection against passenger or third party claims
Up to £15,000 cover for wreck removal and destruction
Pollution related costs and fines
Others being able to use your boat with your permission
Legal liabilities whilst you are in charge of another vessel
Direct access to our claims team and underwriters