Veliko jezero — the Great Lake — is the larger of Mljet's two interconnected saltwater bodies, stretching roughly 5km long and up to 1.5km wide. It sits almost entirely within Mljet National Park, which keeps the shoreline clear of development: no beach bars, no parasols for hire, no jet skis. What you get instead is a wide expanse of warm, clean water with pine-covered hills dropping straight to the water's edge and the monastery island as a focal point in the middle. Swimming is possible from most of the shore, though the clearest entry points are near Soline and around Mali Most.
Kayaking is the best way to explore the lake's full extent. Rentals are available in Polače and Pomena, and the route around the perimeter passes several places where the forest comes to the edge of the water and you can pull ashore in complete quiet. The lake connects to the smaller Malo jezero through a narrow channel at Mali Most, and to the sea through the Soline Channel — so technically the water circulates, though slowly. In practice it behaves like a sheltered cove: the salt content is close to seawater, the temperature is several degrees warmer, and the currents are gentle enough that even basic swimmers feel comfortable.