The Castillo de San Marcos in St. Augustine would be useful. It protects the harbor, with the ocean-facing walls being 19 feet thick. As stone, it would be AV 10 or 11, but given coquina's semi-ductile nature and that it can
absorb 2-4 times as much energy as sandstone, it should probably be a bit higher. There's a well in the courtyard of the fort, which is 100 feet square. The dry moat could be flooded relatively easily. Its only normal entrance is a drawbridge with a barbacan on the opposite side. The parapet is 6 feet high and three feet thick, with 64 piercings for artillery. With around 40 feet of space on top of the walls, there's plenty of room to mount stuff. There are 32 rooms within the walls of the fort.
It's a bit close to the Sea Lord's territory in Jacksonville, but against your typical marauders it would be an incredibly tough nut to crack.