In 1967 Cate Archer was assigned the task of sneaking on board a Finnish freighter that UNITY believed was linked with H.A.R.M. Just after this freighter had left the Bremen docks, it sank into the North Sea (thanks to Tom Goodman's efforts to inconvenience the bad guys.
A day later, Archer and Goodman had sailed out to the North Sea above the wreckage of the freighter so that Archer could dive down and retrieve the captain's log and shipping manifest. Archer was ambushed by H.A.R.M. divers attempting to retrieve the same items.
North Sea on Wikipedia