The large shallow inlets and bays feature comprises the embayment of St Brides Bay, the marine inlet (or ria) of the Milford Haven waterway and peripheral embayments including: Whitesands Bay; South Haven, Skomer; Gateholm – West Dale bays; Freshwater West.

St Brides Bay seabed generally slopes gradually westwards from the sediment flats at its eastern shore. Depth in the outer parts of the bay reaches 50m. The Bay has a wide range of sediment types. These different sediments support a wide variety of species including long-lived animals buried within and living on the sediment surface.

The shallow inlet which is the outer section of the Milford Haven waterway contains species of particular interest - eelgrass and maerl. Eelgrass (Zostera spp.) is the only subtidal British marine flowering plant and is rare in Wales. The plants form very productive beds which stabilise mobile sediment and provide shelter for fish. Known subtidal beds within the SAC include North Haven, Skomer and Littlewick Bay within the Milford Haven waterway. Maerl is a chalky red seaweed that interlocks to form a loose lattice structure which provides a useful place to live for other species. Maerl beds are present along the north side of the waterway.