Having to estimate habitable volume available, we can make an approximation and multiply the surface of the Earth for a hundred metres. The amount you get is about 15 million cubic kilometers. Seems like a huge number, but if you compare it with that oceanic 1.5 billion cubic kilometers (about a hundred times more) disappears in comparison.
In short, the sea has much more space than is available on the Mainland, and mankind has always fantasized about what it might be in these vast spaces. In fairness, some things we discovered, and they are frightening even in just their natural version. Combining natural monsters with stories that have always been told about sea monsters (such as kraken, leviathans and relatives of godzilla), should not come as a surprise that the seas of Aegis Aurea are disreputable places.
The oceans, left be themselves for centuries, have repopulated with countless animals, and magical creatures of all shapes and sizes. Fast moving marine piranha in vast herds that devour everything and giants so big to topple the mind, that could devour a whale in one bite. For this reason the Empire was basically forced into the Mediterranean, except timid attempts to explore the African coast and Spain without ever straying too far from the shore. Given the tendency of the ships to disappear unceremoniously, often these explorations are accomplished with smaller ships and of little tactical value to limit the extent of damage, while the main fleet vessels are reserved for the Mediterranean.
Within the Mediterranean, the situation is quieter. Somehow the man has taken back possession of the surface, and as long as you stay on it the problems are limited. Places like Scylla and Charybdis places continue to be unsafe to cross, especially with small boats, but most of the time ships can stay quiet knowing that nothing will attack them. Or rather, nothing magical, because piracy is a business that’s always very active.
What happens though when one drops below the surface? Much of the Mediterranean seabed is effectively unknown territory. Outside of the shallow regions, such as the Adriatic, Mediterranean can reach depths, from 2000 up to 5000 metres in front of the Peloponnese peninsula. The Mediterranean is in effect divided into two regions, East and West, separated by the Strait of Sicily (the shallow region between Sicily and Africa).
From historical sources find it is known for sure that in the past were built huge submarine structure,real submerged cities. The Empire is trying to find them for a long time, but without success. Many missions have been launched for this purpose, but have not found anything or have never come back. At the same time they tried to colonize the shallower depths, so far with mixed results. At present all these colonies are uninhabited except for brief periods.
In short, in the Mediterranean sea there is still much to discover, well more than you might think. Then, sooner or later, there will be oceans …