Onigashima is the 16th island in the Second Sea
Where NPC's like Kidd and Kidd's pirates are found here along with some of the beast pirates
Kidd's quest gives B$1.5B
While Kid pirates quest gives B$400M
Island is mostly useful for farming money Getting Kidd's drops and not much else but I would suggest for farming cash
on this island you can also find Andre who gives you the quest for kings Haki
This island was renamed to "Skull Island" in the recent anti-copyright update.