Mermaid Cove (人魚の入り江, Ningyo no Irie?) is the place where the mermaids gather and socialize. They live in a coral mansion on the sea floor of Fishman Island, and the employees of Mermaid Cafe live in a same dorm in the cove. It resides in Ryugu Kingdom.

http://images.wikia.com/onepiece/images/4/4a/Mermaid_Cove.png