The area that surrounds the Cíes archipelago is one of the richest marine ecosystems on the Galician coast. One of the most important species found here are brown algae (Phaeophyta) particularly of the genus Laminaria and Saccorhiza.

The movement of the water as it enters and leaves the bay (ría) and the mixture of salt and fresh water favours the growth of micro-organisms which are the main food source for marine species. Therefore, in the surroundings of the islands there is a wide diversity of crustaceans, fish, molluscs, etc.

The underwater species include lobster (Hommarus gammarus), spider crabs (Maja squinado), octopus (Octupus vulgaris), and so on.

Around the cliffs there is a large amount of goose barnacles (Pollicipes cornucopia) and mussels (Mytilus).

In the rocky areas within the islands there are numerous sea urchins (paracentrotus lividus), and large forests of anemones (Anemonia sulcata).

The most sheltered beaches are home to a multitude of bivalve molluscs and fish species such as sole (Solea lascaris), turbot (Psetta maxima), etc.

Frequent visitors to these waters also include dolphins (Delphinidae), whales (Mysticetiy), seals (Phocidae), and others.

Underwater archaeological remains are also to be observed.

Underwater fishing and gathering shellfish is forbidden.