This sunken object appears to lie between AICW markers #36 and #37 in the South Amelia River section of the Waterway, south of Fernandina Beach.
Local Notice to Mariners:
The Coast Guard has received a report of an unknown submerged metallic object at a reported depth of 3.5 ft in approximate location 30-34.148N 081-28.409W. All mariners are advised to transit the area with caution. [Ref: JAX BNM 331-10] Chart 11488