Merovingian golden bees
No, not the kind from Zelda: these gold-and-garnet bees, which date to about the fifth century, were among five hundred originally found in a Merovingian tomb that was uncovered by a stonemason in 1653. They were passed among the royals, given to the Sun King, and housed in the Louvre and the National Library, until they were stolen in the 19th century. Eventually, the police fished just two of these storied bees from the river... which means that there are likely a lot more waiting to be found.
A Gallic ship
Arthur Forgeais, a famous collector and antique seller, was known for finding all sorts of objects in and around the Seine throughout the second half of the 19th century. One of his most famous discoveries happened in 1862: a Gallic boat at the tip of the Île de la Cité. Dating back to the earliest Parisians, the artifact was quite the find for the time... so much so that Forgeais offered it to the Empress of France.