Need help for a class project.While snorkeling in Islamorada we saw small underwater mounds of sand about a foot around and 4 to 6 inches high.They had a entrance hole about the size of a dime.Very white sand was used.They were very close to shore in about 3 feet of water.Does anyone know what these could be??Also while snorkeling we were surrounded by thousands and thousands of small 2-3 inch silverish fish,does anyone know what these could be??We were behind the Hampton Inn in Islamorada.Thanks for your help