The Tremiti Islands are part of a marine protected area, and the underwater world here is considerably richer than the unprotected stretches of the Adriatic coast. Posidonia seagrass meadows shelter octopus, sea bream, grouper, and moray eel; the limestone cliffs drop steeply below the surface and are colonised by gorgonian corals, sponges, and encrusting organisms that make for vivid diving. Visibility in calm conditions is often 20–30 metres.
A dive centre on San Domino offers guided dives for certified divers and introduction dives for beginners. Snorkelling is excellent from many points around the islands, particularly around the rocks on the southern side of San Domino where the water is deep quickly and the fish life is dense. Equipment hire is available on the island. The marine reserve status means fishing is restricted in some zones, which has allowed fish stocks to recover — the underwater life here is noticeably more abundant than comparable spots elsewhere on the Puglia coast.