This is a very small and under-visited attraction. It consists of 2 lakes, one semi-circular, the other rectangular. These used to be the naval and commercial harbours of ancient Carthage.
We spent most of our time on the island in the middle of the semi-circular lake. You actually need a global ticket to get in (something which lets you into nearly all of Carthage's museums and attractions), but this cannot be bought on site. It can most reliably be bought at the Carthage Museum, some 20 minutes walk away.
The island has had several layers exposed, both of the ancient Carthaginian docks and slipways, and the later Roman temple built on top of it. It was deserted when we visited, and for nearly an hour we were the only people exploring the site. Lots of ruins to clamber over and explore. The caretaker was kind enough to open up the little room that houses the reconstruction models. These depict both the Carthaginian naval harbour and the Roman harbour. Very popular with young children.