The sunken quarter was once part of Yalahar, but the most of the area broke from the land and sunk into the sea. There is still some land above water so that you can walk from the main gate or side gates to the adjacent quarters.
You need a Helmet of the Deep to go underwater.
You should bring all the food you will need, because shrimp is semi-rare and is found in small quantities. You should have area runes or spells, in case you are caught by fast quaras, like the mantassins. Medium levels: you should hunt the constrictors and mantassins only. The loot certainly pays the expenses, specially if you can carry the heavier items, like two-handed swords and halberds. However, it is not a great place to hunt for experience, because it takes long to go from one spawn to another. You will not make much more than 28 Kxp/h, unless you use area runes or spells frequently.