As I pointed out in Saint Patrick of Glastonbury?, you can’t begin to understand the old mediaeval stories about saints if you imagine that people in those days had the same understanding of history—and obsession with literal truth—that we do today. There were no printed books before the 15th century, and handwritten records were few in number, terse to the point of being cryptic, and not widely distributed. So, to put it bluntly, things got made up. As in the case of The Buddha in Mediaeval Europe, stories from one tradition got adapted to another tradition. And as with Saint Patrick at Glastonbury, there was another motivation – the lucrative pilgrimage trade. People would pay good money to see the relics of saints, as long as there was a reasonable explanation of how the relics got to that particular place.
The church is dedicated to “Saintes Maries” in the plural. In one of the most important scenes in the Bible, three women—Mary Magdalene, Mary Jacobe and Mary Salome—go to the tomb of Jesus and find it empty. According to mediaeval legend, these three women later ended up being shipwrecked on the coast of France. Tradition ascribes this to the site of present-day Saintes-Maries-de-la-Mer. Oddly, however, the church’s most prominent relics are not those of the three Marys but another saint altogether: Saint Sara.
another mediaeval saint, this isn’t as far-fetched as it sounds. There was dialogue between different cultures in mediaeval times, but people would have interpreted things they heard in terms of their own culture – rather than trying to understand the other person’s culture in the way that a modern scholar or internet user would.
For a really far-fetched theory, we have to turn as usual to Dan Brown and The Da Vinci Code. This confidently asserts that Sara (Anglicized as Sarah) was the daughter of Mary Magdalene and... well, you can guess the rest!