Photo was taken in the Gulf near the Bon Secour National Wildlife Refuge. It seems almost like the discarded flag was staged, but wow, what a powerful message! Photo credits to Press-Register/Ben Raines.
WRIT OF MAINPRIZE, English law. A writ directed to the sheriff (either generally, when any man is imprisoned for a bailable offence, and bail has been refused; or specially, when the offence or cause of commitment is not properly bailable below) commanding him to take sureties for the prisoner's appearance, commonly called mainpernors, and to set him at large. 3 B]. Com. 128. Vide Mainprize.
A Law Dictionary, Adapted to the Constitution and Laws of the United States. By John Bouvier. Published 1856.