Cormorants feed almost entirely on fish. They will typically, individually consume up to half a kilo of fish a day. In fresh water their diets may be more varied. Cormorants ‘duck dive’ from the water’s surface and in addition to their webbed feet, use their short wings to propel themselves downwards to an average depth of 10 metres and catch fish using the sharp hook at the end of their beak.
For a full guide on what cormorants eat, check out this article.