The Pembrokeshire Marine SAC includes not only a substantial part of the Pembrokeshire coastline, but also the Milford Haven Waterway. It is not the islands themselves that are included within the site, but the surrounding shore, seabed and sea.

The landward boundary of the Pembrokeshire Marine SAC is generally at the mean low water mark of ordinary tides. However, it is Government policy that this boundary extends up to the mean high water mark of ordinary tides on areas of shore that have been designated as Sites of Special Scientific Interest (SSSI) and that are included within the SAC.

The Pembrokeshire Marine SAC overlaps with other important European sites. These are called Special Protection Areas (SPA's) that are specifically designated for the protection of birds (under the 1979 EC Bird's Directive). SPA's that occur within the SAC and contain an intertidal element include Skokholm and Skomer, Grassholm, the St David's Peninsula Coast and Ramsey. SAC's and SPA's are both also known as European Marine Sites.