There is a small opening to a cave system on the outer edge (I think around the south-west corner). You should see some freshwater fish around it. There are also usually ruins somewhere outside the freshwater area, and there is a passageway inside the well there. Admittedly, it took me some time to find it the first time around too. Best of luck!
How did you get into that landmass/cave system? I'm stuck
There is a small opening to a cave system on the outer edge (I think around the south-west corner). You should see some freshwater fish around it. There are also usually ruins somewhere outside the freshwater area, and there is a passageway inside the well there. Admittedly, it took me some time to find it the first time around too. Best of luck!