Night Probe by Clive Cusser

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 foundand the one American who can do the job is Dirk Pitt.
 
But in London, a daring counterplot is being orchestrated to see that the treaty is never implemented. Brian Shaw, a master spy who has often worked hand in hand with American agents, now confronts his most challenging command. Pitts mission: Raise the North American Treaty. Shaws mission: Stop Pitt.