Isla Cruces is a fictional island. The relevant scenes were actually filmed in Dominica and Great Exuma, The Bahamas.
In Dead Man's Chest, Isla Cruces (Crosses Island) is a tropical island where Davy Jones buried the Dead Man's Chest containing his beating heart. The island appears to have been abandoned. The previous occupants are unknown, but the partially remaining stone buildings indicate some European-type colony, probably Spanish, was once established there.
It is not revealed in the film why Davy Jones buried the Dead Man's Chest on Isla Cruces. However, the film's writers, Ted Elliott and Terry Rossio, imply in the Pirates of the Caribbean: Dead Man's Chest DVD audio commentary, that Jones chose it because it is a plague island that remains deserted.
Read more about this topic: List Of Locations In Pirates Of The Caribbean