Book 2 in the Troy Bodean Tropical Adventure series.
"Troy Bodean is .. flying a seaplane for his brother’s Key West tourist ferry to Fort Jefferson. And then he sees it... a shadow deep in the water. Dreams of treasure, fortune, and fame begin to fill his head. He launches a secret salvage operation to bring up the mysterious find. And that's when everyone started trying to kill him."
Showing up towards the end of this adventure is Chris Collins, Deputy Director of the CIA to lend a hand resolving the situation.