This assessment provides a general synthesis of the results of three surveys, which together have covered all areas of the subtidal Harbour bed. It is also designed to serve as a baseline, according to which any changes concerning species or areas of particular interest can be identified by resurveying. The surveys were conditioned by the NCC, with additional funding and practical support from various authorities, industries and individuals concerned with the Harbour. The primary objective of this work is to provide an independent source of factual information of practical value in the context of Environmental Management within the Harbour. To this end, the report has been prepared for users concerned with various aspects of the Harbour and representing a wide range of scientific experience. Seperate sections, written in non-scientific and scientific language respectively, contain descriptions of the hydrography, and the substrates and biological communities of the Harbour bed. These have also been considered with respect to their environmental significance, and whether they have been affected by human activities, particularly navigation, reclamation, fisheries, pollution, or the introduction of alien species. The interdependences between natural and human aspects of the subtidal system are considered in the context of Environmental Management, and particularly the prospects for maintaining or improving natural environmental quality. Records currently considered sensitive have been removed from this dataset.