Glenn Cole of central California has sent me this answer:
The "ship of fools" comes from Sebastian Brandt's famous "Ships of Fools" published in 1494 which had as its theme the shipping of fools from their native land of Swabia to a fictional land of Fools. This story comes from the 15th century Germany where river boats (hell boats) were used to imprison the insane.

And Judy Finer of San Francisco sent this:
Re "ship of fools" - the reference I find cites a late 1400's book by Sebastian Brant, maybe in Latin or German (?) Its theme was the shipping off of fools of all kinds from their native lands to the "Land of the Fools." in early 1500 there was an English verse translation by Alexander Barclay. The Brant is said to have been very popular because of its "spirited illustrations" - My oh my. I wonder if there is a source to see these illustrations? Even without, my visual imagination soars. Plus I can readily nominate some key people who ought to be shipped, particularly during this Presidential campaign year here...

Thank you Glenn and Judy.