Miningfag here. The land rises uo out of the water so yes it is rock, coral, etc underneath. Its not water-permeable sand throughout or it would be a sinking or floating land mass. I have mined on Admiralty Island in Alaska where the mine is close to the ocean yet starts at 980' ASL and goes as deep as -I 500' BSL. Not an issue unless you encounter a linked cavern or permeable deposit linking ocean to excavation.