Kefalonia is balancing on the edge of European continental plate, which means at its west lie some of the deepest waters of Mediterranean while at the East there is a protected, shallow sea that expands from the gulf of Patras to the Acheloos river delta and the Echinades islands archipelagos. The biodiversity in this environment is so huge and important that most of Kefalonian coastline is protected under the Natura 2000 network.

Apart from fish and aquatic life that represents the most of Mediterranean ecosystems, Kefalonia is home to the endangered Monk seal Monachus monachus, the Loggerhead Sea Turtle Carreta carreta, (with Skala and Katelios among the most important hatching shores), but also to Short Beaked Dolphins Deplhinus delfis, Bluefin Tuna Thunnus thynnus, Devil Manta Rays Mobula mobular, while Sperm whales Physeter macrocephalus and Fin whales Balaenoptera physalus are frequent visitors.

Kefalonia has also a breathtaking coastline with underwater walls, caverns and caves, extended reefs, Sea grass Posidonia oceanica fields and some really deep waters home to Mediterrannean Red Coral Corallium rubrum. The island also holds few impressive lake Caves such as Melissani, Hiridoni, Karavomilos and more, with some of them accessible to experienced cave divers.

The island is situated in the middle of shipping lanes of Adriatic Sea between Greece and Italy, at the line that separated Western from Eastern Mediterranean Sea. No wonder why the most famous sailor of all times, Ulysses, comes from the neighborhood.. The sea around the island holds a treasure of ancient shipwrecks from prehistoric times 2750-2000 BC, to the late Roman and Byzantine times of 600 AD. The divers could find a variety of amphorae, stone or lead anchors, ballast stones and pieces of sculptures and wood from the ships in an underwater museum that is limited only by the deep water that surrounds the island.

Also Kefalonia was a theater of WW II, not only between the Axis and the Allies, but also between the Nazis and their former Italian allies. Had it not been for the deep waters around the island there would be more wrecks accessible to divers of the Junkers 88 and Bristol Beaufighter and Blenheim bombers, the SS Ardenna POW ship, the F 495 Naxos Landing Craft and the most iconic of wrecks the HMS Perseus submarine.

Perseus submarine is not only among the most impressive wreck dives a diver can experience, but has an amazing history as well. The vessel was on combat patrol in December 1941, and while surfacing at night hit an Italian naval mine and sunk roughly a mile off the coast of Kefalonia. From the crew of 59 only one, the Royal Navy leading stoker John Capes managed a daredevil escape from a depth no one has attempted before, swam his way to Katelios and with the aid of locals escaped capture from Italian and German occupation forces and finally transferred in Turkey. While legendary in Royal Navy, almost nobody believed his adventure until in 1997 a team of Greek divers located the submarine and verified details of his described escape.