Fish Isle is a Major Town in Fishman Island. Like the rest of the island, it is inhabited by Fishmen.
It is completely in ruins, but prior to being overrun it contained several medium sized buildings. The fact that part of the town is surrounded by walls suggests that it was a smaller outpost that was finally found to be worth investing into. It was probably expanding rapidly before being abandoned quite abruptly.
This location spawns a lot of low level loot.