This entertaining adventure novel is Book 1 in The Deepscape Files series by author Sean Kingsley.

Publisher
One summer, a London aristocrat, a group of US surfers, an impulsive Andalusian programmer, and an introverted Oxford scholar embark on a journey along America’s East Coast. In Key West, they meet a treasure hunter who captivates them with tales of $250 million in lost Spanish gold beneath the Straits of Florida.
However, the hunt for treasure leads to conflict as Key West wreck hunters claim ownership, Spain defends its heritage, and an American Indian tribe seeks justice for a historical grievance. From Florida to Europe, the search spirals into a high-stakes battle over sunken riches, secret deals and freedom of the seas.