Their roof has an open mike for any musician or busker, and poetry slams and readings there can sometimes attract hundreds. Success hasn’t much changed the classic canalboat shabbiness, with assorted junk on roof and gunwales, and odds and ends of potted plants and bags of smokeless coal (for the cozy stove below decks).

Mr. Screech and Mr. Privett have a collection curated to appeal to the many tourists who pass by, but also to a hard-core of radical and leftist readers; Noam Chomsky and Jack London are big sellers.

“Basically my formula is half are the best books that I’ve read, and the other half are ones I want to read,” Mr. Privett said.

For a while they were selling “I Love Dick”, the feminist memoir-novel by Chris Kraus — before it became a television hit with Kevin Bacon and Kathryn Hahn — from the outside book racks. “People kept stopping to take selfies with that book,” Mr. Privett said. “Well, what do you expect, with a title like that?”

Many people have suggested that the men sell trinkets and cards, or especially coffee and drinks, but they’ve resisted such commercialization.

“I didn’t study English literature for three years to serve coffee,” Mr. Privett said. Mostly, he said, they are in it for the “lifestyle perks” — for example, “an unlimited supply of books,” which in turn leads to limitless book conversations.

“Do you know how much I’d have to earn to have this many books?” he said.

This year there was finally enough income for other perks. Mr. Privett was able to take his daughter Megan, who is 15, on a trip to New York with him (she lives with her mother; her parents never wed and no longer live together). “For years I had been promising her a trip to New York and finally I was able to do it,” he said. “I was able to be a role model for her, and show her if you follow your dream and work hard, you can do it.”

Word on the Water is not taking any chances with success. There’s a bookcase blocking the door to the sea toilet now. Instead, staff and customers use the loos at an upscale pub nearby. Nobody seems to mind.