One of the real pleasures of “pay what you can” Wednesdays after 4pm at Boston’s Museum of Fine Arts is the luxury of entering the collection for just an hour or two, selecting one exhibit or a few paintings to see and then going on your way. Always on my agenda is this painting, one of my favorites: John Singleton Copley’s Watson and the Shark. Based on a true story (as told to the painter by his friend, the survivor Brook Watson), it records a 1749 attack in Havana harbor. That Copley had never been to Havana doesn’t lessen the impact. Neither does his never having seen a shark (judging from his portrayal of the killer fish with lips and maybe even lipstick.) Why the man in the water is naked, who knows? But it does lend a classical air to the story, based, I was told, on the statue of the “Borghese Gladiator” in the Louvre.