What does Jacob see see underwater as they approach the island?

From the text:

As we drew nearer the cliffs, I began to notice odd shapes lurking underwater. A passing crewman caught me leaning over the rail to stare at them and said, “Never seen a shipwreck before, eh?”

I turned to him. “Really?”

“This whole area’s a nautical graveyard. It’s like the old captains used to say — ‘Twixt Hartland Point and Cairnholm Bay is a sailor’s grave by night or day!’ ”