I checked out all of these and went with Boat US for my '71 Seafari. $297 yearly including trailer and $5K contents and accessories coverage, plus $104 for unlimited salt and freshwater towing and up to 150 miles of trailer towing. (West Marine Associate discount)

State Farm flat would not insure it, even with a survey, (said boat too old).
I also will not use Progressive but because of their support of Planned Parenthood.
Allstate required a survey before they'd even talk to me even for an agreed value policy, and
Great Lakes wanted 20% more money and a survey within 90 days if I wanted to cover the boat, motor, trailer and contents for more than $5K total value.