Night Probe! (Dirk Pitt #6) by Clive Cussler

In the midst of an international crisis, Heidi Milligan, a beautiful, brilliant American naval commander, accidentally discovers an obscure reference to the long-buried North American Treaty, a precedent-shattering secret pact between the United States and Great Britain.

The President believes that the treaty offers the single shot at salvation for an energy-starved, economically devastated nation, but the only two copies plummeted into the watery depths of the Atlantic in twin disasters long ago. The original document must be found—and the one American who can do the job is Dirk Pitt.