Large shallow inlets and bays are large indentations of the coast, generally more sheltered from wave action than the open coast. They are relatively shallow (with water less than 30 m over most of the area), and generally have much lower freshwater influence compared to estuaries. They comprise an interdependent mosaic of subtidal and intertidal habitats.

The large shallow inlets and bays feature in Carmarthen Bay and Estuaries SAC is Carmarthen Bay. Carmarthen Bay, is an extensive shallow bay, partially bound by rocky outcrops, mostly occupied by soft sediments. It extends from Tenby and Caldy Island in the West to Worm’s Head on the Gower peninsula in the east.

Throughout the bay physical conditions vary considerably. Salinity varies from low (at the estuaries) to fully marine. There are gradients in wave action from sheltered to exposed. Strong tides sweep exposed headlands whilst other areas are sheltered from currents. There is a wide range of seabed types, including mud, sand and rock, although the majority of the seabed is sandy. The sediment supports a large number of species, including bivalve molluscs, worms, burrowing urchins, brittlestars and sand-stars.